<?xml version="1.0" encoding="utf-8"?>
<!--Generated by DFIN XBRL Instance Document - http://www.dfinsolutions.com/ - Version R3.0 - on 01-August-2022 [09:43:04] {PM}-->
<linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#CoverPage" roleURI="http://unitil.com/role/CoverPage" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedStatementsOfEarnings" roleURI="http://unitil.com/role/ConsolidatedStatementsOfEarnings" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedBalanceSheets" roleURI="http://unitil.com/role/ConsolidatedBalanceSheets" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedStatementsOfCashFlows" roleURI="http://unitil.com/role/ConsolidatedStatementsOfCashFlows" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedStatementsOfChangesInCommonStockEquity" roleURI="http://unitil.com/role/ConsolidatedStatementsOfChangesInCommonStockEquity" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SegmentInformation" roleURI="http://unitil.com/role/SegmentInformation" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DebtAndFinancingArrangements" roleURI="http://unitil.com/role/DebtAndFinancingArrangements" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#CommonStockAndPreferredStock" roleURI="http://unitil.com/role/CommonStockAndPreferredStock" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RegulatoryMatters" roleURI="http://unitil.com/role/RegulatoryMatters" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#EnvironmentalMatters" roleURI="http://unitil.com/role/EnvironmentalMatters" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SummaryOfSignificantAccountingPoliciesTables" roleURI="http://unitil.com/role/SummaryOfSignificantAccountingPoliciesTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SegmentInformationTables" roleURI="http://unitil.com/role/SegmentInformationTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DebtAndFinancingArrangementsTables" roleURI="http://unitil.com/role/DebtAndFinancingArrangementsTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#EnvironmentalMattersTables" roleURI="http://unitil.com/role/EnvironmentalMattersTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RetirementBenefitObligationsTables" roleURI="http://unitil.com/role/RetirementBenefitObligationsTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" roleURI="http://unitil.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfGasAndElectricOperatingRevenueDetail" roleURI="http://unitil.com/role/ComponentsOfGasAndElectricOperatingRevenueDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfAccruedRevenueDetail" roleURI="http://unitil.com/role/ComponentsOfAccruedRevenueDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfExchangeGasReceivableDetail" roleURI="http://unitil.com/role/ComponentsOfExchangeGasReceivableDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfGasInventoryDetail" roleURI="http://unitil.com/role/ComponentsOfGasInventoryDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RegulatoryAssetsDetail" roleURI="http://unitil.com/role/RegulatoryAssetsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RegulatoryLiabilitiesDetail" roleURI="http://unitil.com/role/RegulatoryLiabilitiesDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#FairValueOfMarketableSecuritiesDetail" roleURI="http://unitil.com/role/FairValueOfMarketableSecuritiesDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfEnergySupplyObligationsDetail" roleURI="http://unitil.com/role/ComponentsOfEnergySupplyObligationsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DividendsDeclaredPerShareDetail" roleURI="http://unitil.com/role/DividendsDeclaredPerShareDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SignificantSegmentFinancialDataDetail" roleURI="http://unitil.com/role/SignificantSegmentFinancialDataDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DetailsOnLongTermDebtDetail" roleURI="http://unitil.com/role/DetailsOnLongTermDebtDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DetailsOnLongTermDebtParentheticalDetail" roleURI="http://unitil.com/role/DetailsOnLongTermDebtParentheticalDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#EstimatedFairValueOfLongTermDebtDetail" roleURI="http://unitil.com/role/EstimatedFairValueOfLongTermDebtDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DebtAndFinancingArrangementsAdditionalInformationDetail" roleURI="http://unitil.com/role/DebtAndFinancingArrangementsAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#BorrowingLimitsAmountsOutstandingAndAmountsAvailableUnderRevolvingCreditFacilityDetail" roleURI="http://unitil.com/role/BorrowingLimitsAmountsOutstandingAndAmountsAvailableUnderRevolvingCreditFacilityDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DebtAndFinancingArrangementsClassificationOfTheCompanyLeaseObligationsDetail" roleURI="http://unitil.com/role/DebtAndFinancingArrangementsClassificationOfTheCompanyLeaseObligationsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#FutureOperatingLeasePaymentObligationsAndFutureMinimumLeasePaymentsUnderCapitalLeasesDetail" roleURI="http://unitil.com/role/FutureOperatingLeasePaymentObligationsAndFutureMinimumLeasePaymentsUnderCapitalLeasesDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#CommonStockAndPreferredStockAdditionalInformationDetail" roleURI="http://unitil.com/role/CommonStockAndPreferredStockAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RestrictedStockUnitsIssuedDetail" roleURI="http://unitil.com/role/RestrictedStockUnitsIssuedDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RegulatoryMattersAdditionalInformationDetail" roleURI="http://unitil.com/role/RegulatoryMattersAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#EnvironmentalMattersAdditionalInformationDetail" roleURI="http://unitil.com/role/EnvironmentalMattersAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#CompanysLiabilityForEnvironmentalObligationsDetail" roleURI="http://unitil.com/role/CompanysLiabilityForEnvironmentalObligationsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DifferencesBetweenProvisionsForIncomeTaxesAndProvisionsCalculatedAtStatutoryFederalTaxRateDetail" roleURI="http://unitil.com/role/DifferencesBetweenProvisionsForIncomeTaxesAndProvisionsCalculatedAtStatutoryFederalTaxRateDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#IncomeTaxesAdditionalInformationDetail" roleURI="http://unitil.com/role/IncomeTaxesAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#KeyWeightedAverageAssumptionsUsedInDeterminingBenefitPlanCostsAndObligationsDetail" roleURI="http://unitil.com/role/KeyWeightedAverageAssumptionsUsedInDeterminingBenefitPlanCostsAndObligationsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ComponentsOfRetirementPlanCostsDetail" roleURI="http://unitil.com/role/ComponentsOfRetirementPlanCostsDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RetirementBenefitObligationsAdditionalInformationDetail" roleURI="http://unitil.com/role/RetirementBenefitObligationsAdditionalInformationDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#RetirementBenefitObligations" roleURI="http://unitil.com/role/RetirementBenefitObligations" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedBalanceSheetsParenthetical" roleURI="http://unitil.com/role/ConsolidatedBalanceSheetsParenthetical" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#ConsolidatedStatementsOfChangesInCommonStockEquityParenthetical" roleURI="http://unitil.com/role/ConsolidatedStatementsOfChangesInCommonStockEquityParenthetical" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SummaryOfSignificantAccountingPolicies" roleURI="http://unitil.com/role/SummaryOfSignificantAccountingPolicies" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#SummaryOfSignificantAccountingPoliciesPolicies" roleURI="http://unitil.com/role/SummaryOfSignificantAccountingPoliciesPolicies" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#AllowanceForDoubtfulAccountsIncludedInAccountsReceivableNetDetail" roleURI="http://unitil.com/role/AllowanceForDoubtfulAccountsIncludedInAccountsReceivableNetDetail" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DividendsDeclaredPerShare" roleURI="http://unitil.com/role/DividendsDeclaredPerShare" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#DividendsDeclaredPerShareTables" roleURI="http://unitil.com/role/DividendsDeclaredPerShareTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#IncomeTaxes" roleURI="http://unitil.com/role/IncomeTaxes" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#CommonStockAndPreferredStockTables" roleURI="http://unitil.com/role/CommonStockAndPreferredStockTables" />
<roleRef xlink:type="simple" xlink:href="utl-20220630.xsd#IncomeTaxesTables" roleURI="http://unitil.com/role/IncomeTaxesTables" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
<arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/notAll" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#notAll" />
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/CoverPage">
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentInformationLineItems" xlink:label="loc_dei_DocumentInformationLineItems_489493" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentInformationTable" xlink:label="loc_dei_DocumentInformationTable_489490" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentInformationTable_489490" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_489491" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_489490" xlink:to="loc_us-gaap_StatementClassOfStockAxis_489491" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_489492" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_489491" xlink:to="loc_us-gaap_ClassOfStockDomain_489492" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_489492_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_489491" xlink:to="loc_us-gaap_ClassOfStockDomain_489492_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityInteractiveDataCurrent" xlink:label="loc_dei_EntityInteractiveDataCurrent_489494" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityInteractiveDataCurrent_489494" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentQuarterlyReport" xlink:label="loc_dei_DocumentQuarterlyReport_489495" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentQuarterlyReport_489495" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentTransitionReport" xlink:label="loc_dei_DocumentTransitionReport_489496" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentTransitionReport_489496" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_489497" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentPeriodEndDate_489497" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_489498" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityRegistrantName_489498" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityFileNumber" xlink:label="loc_dei_EntityFileNumber_489499" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityFileNumber_489499" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityTaxIdentificationNumber" xlink:label="loc_dei_EntityTaxIdentificationNumber_489500" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityTaxIdentificationNumber_489500" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityIncorporationStateCountryCode" xlink:label="loc_dei_EntityIncorporationStateCountryCode_489501" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityIncorporationStateCountryCode_489501" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityShellCompany" xlink:label="loc_dei_EntityShellCompany_489503" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityShellCompany_489503" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_489504" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityFilerCategory_489504" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntitySmallBusiness" xlink:label="loc_dei_EntitySmallBusiness_489505" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntitySmallBusiness_489505" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityEmergingGrowthCompany" xlink:label="loc_dei_EntityEmergingGrowthCompany_489506" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityEmergingGrowthCompany_489506" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityAddressAddressLine1" xlink:label="loc_dei_EntityAddressAddressLine1_489507" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityAddressAddressLine1_489507" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityAddressCityOrTown" xlink:label="loc_dei_EntityAddressCityOrTown_489508" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityAddressCityOrTown_489508" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityAddressPostalZipCode" xlink:label="loc_dei_EntityAddressPostalZipCode_489509" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityAddressPostalZipCode_489509" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityAddressStateOrProvince" xlink:label="loc_dei_EntityAddressStateOrProvince_489510" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityAddressStateOrProvince_489510" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_CityAreaCode" xlink:label="loc_dei_CityAreaCode_489511" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_CityAreaCode_489511" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LocalPhoneNumber" xlink:label="loc_dei_LocalPhoneNumber_489512" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_LocalPhoneNumber_489512" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_TradingSymbol" xlink:label="loc_dei_TradingSymbol_489513" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_TradingSymbol_489513" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_SecurityExchangeName" xlink:label="loc_dei_SecurityExchangeName_489514" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_SecurityExchangeName_489514" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_489516" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_489516" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_489517" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentType_489517" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_Security12bTitle" xlink:label="loc_dei_Security12bTitle_490392" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_Security12bTitle_490392" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_495982" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_AmendmentFlag_495982" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_495983" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentFiscalYearFocus_495983" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_495984" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_DocumentFiscalPeriodFocus_495984" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_495985" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityCentralIndexKey_495985" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_495986" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_CurrentFiscalYearEndDate_495986" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityCurrentReportingStatus" xlink:label="loc_dei_EntityCurrentReportingStatus_906700" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_489493" xlink:to="loc_dei_EntityCurrentReportingStatus_906700" use="optional" order="33" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ConsolidatedStatementsOfEarnings">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451398" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451395" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451398" xlink:to="loc_us-gaap_StatementTable_451395" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_451396" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451395" xlink:to="loc_srt_ProductOrServiceAxis_451396" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_451397" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_451396" xlink:to="loc_srt_ProductsAndServicesDomain_451397" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_451397_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_451396" xlink:to="loc_srt_ProductsAndServicesDomain_451397_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GasMember" xlink:label="loc_utl_GasMember_451401" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_451397" xlink:to="loc_utl_GasMember_451401" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ElectricMember" xlink:label="loc_utl_ElectricMember_451409" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_451397" xlink:to="loc_utl_ElectricMember_451409" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenuesAbstract" xlink:label="loc_us-gaap_RevenuesAbstract_451418" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451398" xlink:to="loc_us-gaap_RevenuesAbstract_451418" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_451421" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_451418" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_451421" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="loc_us-gaap_OperatingExpensesAbstract_451422" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451398" xlink:to="loc_us-gaap_OperatingExpensesAbstract_451422" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_451430" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_OperatingIncomeLoss_451430" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_451431" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_451431" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_451432" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_451432" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_1559671" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_EarningsPerShareBasic_1559671" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_1559672" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_EarningsPerShareDiluted_1559672" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1559673" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1559673" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_1559674" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_1559674" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_451426" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_451426" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization_451427" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization_451427" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilitiesOperatingExpenseTaxes" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseTaxes_451428" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseTaxes_451428" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilitiesOperatingExpense" xlink:label="loc_us-gaap_UtilitiesOperatingExpense_451429" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_UtilitiesOperatingExpense_451429" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_451433" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_451433" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_451434" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_451434" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_451435" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_NetIncomeLoss_451435" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CostsAndExpenses" xlink:label="loc_us-gaap_CostsAndExpenses_1402580" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_451422" xlink:to="loc_us-gaap_CostsAndExpenses_1402580" use="optional" order="24" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ConsolidatedBalanceSheets">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451469" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451466" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451469" xlink:to="loc_us-gaap_StatementTable_451466" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_451467" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451466" xlink:to="loc_us-gaap_StatementClassOfStockAxis_451467" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451468" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451467" xlink:to="loc_us-gaap_ClassOfStockDomain_451468" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451468_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451467" xlink:to="loc_us-gaap_ClassOfStockDomain_451468_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451469" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalizationLongtermDebtAndEquityAbstract" xlink:label="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_451547" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_451547" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_451532" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_451547" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_451532" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_451545" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_451545" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShortTermBorrowings" xlink:label="loc_us-gaap_ShortTermBorrowings_451506" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_ShortTermBorrowings_451506" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaap_AccountsPayableCurrent_451505" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_AccountsPayableCurrent_451505" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesCurrent" xlink:label="loc_us-gaap_LiabilitiesCurrent_451512" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_LiabilitiesCurrent_451512" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtCurrent" xlink:label="loc_us-gaap_LongTermDebtCurrent_451507" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_LongTermDebtCurrent_451507" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityCurrent" xlink:label="loc_us-gaap_RegulatoryLiabilityCurrent_451508" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_RegulatoryLiabilityCurrent_451508" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnergyMarketingContractLiabilitiesCurrent" xlink:label="loc_us-gaap_EnergyMarketingContractLiabilitiesCurrent_451509" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_EnergyMarketingContractLiabilitiesCurrent_451509" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_451511" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_451511" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccruedEnvironmentalLossContingenciesCurrent" xlink:label="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_546282" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_546282" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestPayableCurrent" xlink:label="loc_us-gaap_InterestPayableCurrent_906709" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_451545" xlink:to="loc_us-gaap_InterestPayableCurrent_906709" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaap_StockholdersEquityAbstract_451548" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_StockholdersEquityAbstract_451548" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_451535" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_451548" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_451535" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_CommonStockEquity" xlink:label="loc_utl_CommonStockEquity_451536" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_451548" xlink:to="loc_utl_CommonStockEquity_451536" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PreferredStockValue" xlink:label="loc_us-gaap_PreferredStockValue_451537" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_451548" xlink:to="loc_us-gaap_PreferredStockValue_451537" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_451538" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_451548" xlink:to="loc_us-gaap_StockholdersEquity_451538" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStocksIncludingAdditionalPaidInCapital" xlink:label="loc_us-gaap_CommonStocksIncludingAdditionalPaidInCapital_451534" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_451548" xlink:to="loc_us-gaap_CommonStocksIncludingAdditionalPaidInCapital_451534" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalizationLongtermDebtAndEquity" xlink:label="loc_us-gaap_CapitalizationLongtermDebtAndEquity_451539" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_CapitalizationLongtermDebtAndEquity_451539" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="loc_us-gaap_CommitmentsAndContingencies_451540" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_CommitmentsAndContingencies_451540" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesNoncurrentAbstract" xlink:label="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesNoncurrent" xlink:label="loc_us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesNoncurrent_451516" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesNoncurrent_451516" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredTaxAndOtherLiabilitiesNoncurrent" xlink:label="loc_us-gaap_DeferredTaxAndOtherLiabilitiesNoncurrent_451518" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_DeferredTaxAndOtherLiabilitiesNoncurrent_451518" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetRetirementObligationsNoncurrent" xlink:label="loc_us-gaap_AssetRetirementObligationsNoncurrent_451520" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_AssetRetirementObligationsNoncurrent_451520" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityNoncurrent" xlink:label="loc_us-gaap_RegulatoryLiabilityNoncurrent_451521" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_RegulatoryLiabilityNoncurrent_451521" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_451522" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_451522" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesNoncurrent" xlink:label="loc_us-gaap_LiabilitiesNoncurrent_451523" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_LiabilitiesNoncurrent_451523" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent" xlink:label="loc_us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent_546285" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesNoncurrentAbstract_451546" xlink:to="loc_us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent_546285" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_451541" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_451526" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_451541" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaap_AssetsAbstract_451527" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451469" xlink:to="loc_us-gaap_AssetsAbstract_451527" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Assets" xlink:label="loc_us-gaap_Assets_451492" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_451527" xlink:to="loc_us-gaap_Assets_451492" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsNoncurrentAbstract" xlink:label="loc_us-gaap_AssetsNoncurrentAbstract_451544" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_451527" xlink:to="loc_us-gaap_AssetsNoncurrentAbstract_451544" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetsNoncurrent" xlink:label="loc_us-gaap_RegulatoryAssetsNoncurrent_451488" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_451544" xlink:to="loc_us-gaap_RegulatoryAssetsNoncurrent_451488" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_451489" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_451544" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_451489" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_451490" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_451544" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_451490" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NoncurrentAssetsOtherThanPropertyPlantAndEquipment" xlink:label="loc_utl_NoncurrentAssetsOtherThanPropertyPlantAndEquipment_451491" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_451544" xlink:to="loc_utl_NoncurrentAssetsOtherThanPropertyPlantAndEquipment_451491" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_451527" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PublicUtilitiesPropertyPlantAndEquipmentGas" xlink:label="loc_utl_PublicUtilitiesPropertyPlantAndEquipmentGas_451480" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_utl_PublicUtilitiesPropertyPlantAndEquipmentGas_451480" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService_451484" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService_451484" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution_451481" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentTransmissionAndDistribution_451481" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentCommon" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentCommon_451482" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentCommon_451482" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress_451483" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress_451483" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation_451485" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation_451485" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_451486" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract_451543" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_451486" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaap_AssetsCurrentAbstract_451542" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_451527" xlink:to="loc_us-gaap_AssetsCurrentAbstract_451542" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_451471" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_451471" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_451478" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_AssetsCurrent_451478" use="optional" order="52" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_451472" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_451472" use="optional" order="53" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnbilledContractsReceivable" xlink:label="loc_us-gaap_UnbilledContractsReceivable_451473" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_UnbilledContractsReceivable_451473" use="optional" order="54" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_451474" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_451474" use="optional" order="55" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageCostInventoryAmount" xlink:label="loc_us-gaap_WeightedAverageCostInventoryAmount_451475" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_WeightedAverageCostInventoryAmount_451475" use="optional" order="56" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InventoryRawMaterialsAndSupplies" xlink:label="loc_us-gaap_InventoryRawMaterialsAndSupplies_451476" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_InventoryRawMaterialsAndSupplies_451476" use="optional" order="57" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PrepaidExpenseAndOtherAssets" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssets_451477" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_451542" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssets_451477" use="optional" order="58" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ConsolidatedStatementsOfCashFlows">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451615" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_451640" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_451640" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_451641" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_451640" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_451641" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_451643" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_451640" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_451643" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_451653" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_451653" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_451654" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_451654" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RepaymentsOfLongTermDebt" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_451647" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_451647" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncreaseDecreaseInGasInventoryFinancing" xlink:label="loc_utl_IncreaseDecreaseInGasInventoryFinancing_451649" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_utl_IncreaseDecreaseInGasInventoryFinancing_451649" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsOfDividends" xlink:label="loc_us-gaap_PaymentsOfDividends_451650" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_us-gaap_PaymentsOfDividends_451650" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncreaseDecreaseInCapitalLeaseObligations" xlink:label="loc_utl_IncreaseDecreaseInCapitalLeaseObligations_451648" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_utl_IncreaseDecreaseInCapitalLeaseObligations_451648" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_451651" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_451651" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromRepaymentsOfShortTermDebt" xlink:label="loc_us-gaap_ProceedsFromRepaymentsOfShortTermDebt_451645" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_us-gaap_ProceedsFromRepaymentsOfShortTermDebt_451645" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_451652" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_451644" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_451652" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451612" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_StatementTable_451612" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxesPaidNet" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_451663" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_451663" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalLeaseObligationsIncurred" xlink:label="loc_us-gaap_CapitalLeaseObligationsIncurred_451665" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" xlink:to="loc_us-gaap_CapitalLeaseObligationsIncurred_451665" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NoncashInvestingActivityAbstract" xlink:label="loc_utl_NoncashInvestingActivityAbstract_451666" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" xlink:to="loc_utl_NoncashInvestingActivityAbstract_451666" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_451672" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_NoncashInvestingActivityAbstract_451666" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_451672" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_451674" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_NoncashInvestingActivityAbstract_451666" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_451674" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InterestPaidNet" xlink:label="loc_us-gaap_InterestPaidNet_451661" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_451659" xlink:to="loc_us-gaap_InterestPaidNet_451661" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451615" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_451635" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_451635" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_451638" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_451638" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_451620" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_451638" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_451620" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization_451619" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_451638" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseDepreciationAndAmortization_451619" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_451617" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" xlink:to="loc_us-gaap_ProfitLoss_451617" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_451616" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherOperatingActivitiesCashFlowStatement" xlink:label="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_451634" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_451634" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_451624" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_451624" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInAccountsPayable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_451628" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_451628" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_451630" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_451630" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ChangeInDeferredRegulatoryAndOtherCharges" xlink:label="loc_utl_ChangeInDeferredRegulatoryAndOtherCharges_451633" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_utl_ChangeInDeferredRegulatoryAndOtherCharges_451633" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInGasImbalanceReceivable" xlink:label="loc_us-gaap_IncreaseDecreaseInGasImbalanceReceivable_451626" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInGasImbalanceReceivable_451626" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInRegulatoryLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInRegulatoryLiabilities_451627" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInRegulatoryLiabilities_451627" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInContractWithCustomerAsset" xlink:label="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_451625" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_451639" xlink:to="loc_us-gaap_IncreaseDecreaseInContractWithCustomerAsset_451625" use="optional" order="36" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ConsolidatedStatementsOfChangesInCommonStockEquity">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451687" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsCommonStock" xlink:label="loc_us-gaap_DividendsCommonStock_451699" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_us-gaap_DividendsCommonStock_451699" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451684" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_us-gaap_StatementTable_451684" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_451688" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451684" xlink:to="loc_dei_LegalEntityAxis_451688" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451689" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_451688" xlink:to="loc_dei_EntityDomain_451689" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451689_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_451688" xlink:to="loc_dei_EntityDomain_451689_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_451690" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451684" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_451690" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_451691" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_451690" xlink:to="loc_us-gaap_EquityComponentDomain_451691" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_451691_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_451690" xlink:to="loc_us-gaap_EquityComponentDomain_451691_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember_451692" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_451691" xlink:to="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember_451692" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_451693" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_451691" xlink:to="loc_us-gaap_RetainedEarningsMember_451693" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_451694" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451684" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_451694" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_451695" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_451694" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_451695" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_451695_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_451694" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_451695_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_451698" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_us-gaap_NetIncomeLoss_451698" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_451701" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_451701" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_CommonStockEquity" xlink:label="loc_utl_CommonStockEquity_451697" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_utl_CommonStockEquity_451697" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_547952" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451687" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_547952" use="optional" order="17" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/SegmentInformation">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452156" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentReportingDisclosureTextBlock" xlink:label="loc_us-gaap_SegmentReportingDisclosureTextBlock_452157" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452156" xlink:to="loc_us-gaap_SegmentReportingDisclosureTextBlock_452157" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452153" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452156" xlink:to="loc_us-gaap_StatementTable_452153" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452154" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452153" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452154" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452155" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452154" xlink:to="loc_us-gaap_ClassOfStockDomain_452155" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452155_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452154" xlink:to="loc_us-gaap_ClassOfStockDomain_452155_default" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DebtAndFinancingArrangements">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452205" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452202" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452205" xlink:to="loc_us-gaap_StatementTable_452202" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452203" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452202" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452203" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452204" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452203" xlink:to="loc_us-gaap_ClassOfStockDomain_452204" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452204_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452203" xlink:to="loc_us-gaap_ClassOfStockDomain_452204_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtDisclosureTextBlock" xlink:label="loc_us-gaap_DebtDisclosureTextBlock_452207" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452205" xlink:to="loc_us-gaap_DebtDisclosureTextBlock_452207" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/CommonStockAndPreferredStock">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451446" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451443" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451446" xlink:to="loc_us-gaap_StatementTable_451443" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_451444" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451443" xlink:to="loc_us-gaap_StatementClassOfStockAxis_451444" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451445" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451444" xlink:to="loc_us-gaap_ClassOfStockDomain_451445" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451445_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451444" xlink:to="loc_us-gaap_ClassOfStockDomain_451445_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:label="loc_us-gaap_StockholdersEquityNoteDisclosureTextBlock_451447" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451446" xlink:to="loc_us-gaap_StockholdersEquityNoteDisclosureTextBlock_451447" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RegulatoryMatters">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452069" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452066" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452069" xlink:to="loc_us-gaap_StatementTable_452066" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452067" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452066" xlink:to="loc_dei_LegalEntityAxis_452067" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452068" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452067" xlink:to="loc_dei_EntityDomain_452068" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452068_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452067" xlink:to="loc_dei_EntityDomain_452068_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesDisclosureTextBlock" xlink:label="loc_us-gaap_PublicUtilitiesDisclosureTextBlock_452070" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452069" xlink:to="loc_us-gaap_PublicUtilitiesDisclosureTextBlock_452070" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/EnvironmentalMatters">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452025" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452022" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452025" xlink:to="loc_us-gaap_StatementTable_452022" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452026" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452022" xlink:to="loc_dei_LegalEntityAxis_452026" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452027" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452026" xlink:to="loc_dei_EntityDomain_452027" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452027_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452026" xlink:to="loc_dei_EntityDomain_452027_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnvironmentalLossContingencyDisclosureTextBlock" xlink:label="loc_us-gaap_EnvironmentalLossContingencyDisclosureTextBlock_452028" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452025" xlink:to="loc_us-gaap_EnvironmentalLossContingencyDisclosureTextBlock_452028" use="optional" order="5" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/SummaryOfSignificantAccountingPoliciesTables">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451907" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ComponentsOfGasOperatingRevenueAndElectricOperatingRevenueTableTextBlock" xlink:label="loc_utl_ComponentsOfGasOperatingRevenueAndElectricOperatingRevenueTableTextBlock_451892" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_utl_ComponentsOfGasOperatingRevenueAndElectricOperatingRevenueTableTextBlock_451892" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_AllowanceForDoubtfulAccountsTableTextBlock" xlink:label="loc_utl_AllowanceForDoubtfulAccountsTableTextBlock_451893" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_utl_AllowanceForDoubtfulAccountsTableTextBlock_451893" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_MarketableSecuritiesTextBlock" xlink:label="loc_us-gaap_MarketableSecuritiesTextBlock_451899" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_MarketableSecuritiesTextBlock_451899" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SupplyCommitmentTableTextBlock" xlink:label="loc_us-gaap_SupplyCommitmentTableTextBlock_451900" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_SupplyCommitmentTableTextBlock_451900" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451904" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_StatementTable_451904" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_451905" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451904" xlink:to="loc_us-gaap_AwardTypeAxis_451905" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451906" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_451905" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451906" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451906_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_451905" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451906_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DeferredCompensationPlanMember" xlink:label="loc_utl_DeferredCompensationPlanMember_451908" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451906" xlink:to="loc_utl_DeferredCompensationPlanMember_451908" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_UnbilledAccountsReceivableTableTextBlock" xlink:label="loc_utl_UnbilledAccountsReceivableTableTextBlock_451894" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_utl_UnbilledAccountsReceivableTableTextBlock_451894" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ScheduleOfOtherAccountsReceivableTableTextBlock" xlink:label="loc_utl_ScheduleOfOtherAccountsReceivableTableTextBlock_451895" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_utl_ScheduleOfOtherAccountsReceivableTableTextBlock_451895" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfUtilityInventoryTextBlock" xlink:label="loc_us-gaap_ScheduleOfUtilityInventoryTextBlock_451896" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_ScheduleOfUtilityInventoryTextBlock_451896" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRegulatoryAssetsTextBlock" xlink:label="loc_us-gaap_ScheduleOfRegulatoryAssetsTextBlock_451897" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_ScheduleOfRegulatoryAssetsTextBlock_451897" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRegulatoryLiabilitiesTextBlock" xlink:label="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTextBlock_451898" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451907" xlink:to="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTextBlock_451898" use="optional" order="14" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/SegmentInformationTables">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452164" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452161" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452164" xlink:to="loc_us-gaap_StatementTable_452161" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTextBlock" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTextBlock_452165" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452164" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTextBlock_452165" use="optional" order="2" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DebtAndFinancingArrangementsTables">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452220" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ScheduleOfFutureMinimumPaymentsForObligationsTableTextBlock" xlink:label="loc_utl_ScheduleOfFutureMinimumPaymentsForObligationsTableTextBlock_452228" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_utl_ScheduleOfFutureMinimumPaymentsForObligationsTableTextBlock_452228" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452217" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_us-gaap_StatementTable_452217" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452218" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452217" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452218" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452219" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452218" xlink:to="loc_us-gaap_ClassOfStockDomain_452219" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452219_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452218" xlink:to="loc_us-gaap_ClassOfStockDomain_452219_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfDebtInstrumentsTextBlock" xlink:label="loc_us-gaap_ScheduleOfDebtInstrumentsTextBlock_452221" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_us-gaap_ScheduleOfDebtInstrumentsTextBlock_452221" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfCarryingValuesAndEstimatedFairValuesOfDebtInstrumentsTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfCarryingValuesAndEstimatedFairValuesOfDebtInstrumentsTableTextBlock_452224" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_us-gaap_ScheduleOfCarryingValuesAndEstimatedFairValuesOfDebtInstrumentsTableTextBlock_452224" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfDebtTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfDebtTableTextBlock_452226" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_us-gaap_ScheduleOfDebtTableTextBlock_452226" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ScheduleOfCapitalLeasesObligationsTableTextBlock" xlink:label="loc_utl_ScheduleOfCapitalLeasesObligationsTableTextBlock_452227" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452220" xlink:to="loc_utl_ScheduleOfCapitalLeasesObligationsTableTextBlock_452227" use="optional" order="9" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/EnvironmentalMattersTables">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452033" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452030" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452033" xlink:to="loc_us-gaap_StatementTable_452030" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfEnvironmentalLossContingenciesBySiteTextBlock" xlink:label="loc_us-gaap_ScheduleOfEnvironmentalLossContingenciesBySiteTextBlock_452034" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452033" xlink:to="loc_us-gaap_ScheduleOfEnvironmentalLossContingenciesBySiteTextBlock_452034" use="optional" order="2" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RetirementBenefitObligationsTables">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_451373" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfAssumptionsUsedTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfAssumptionsUsedTableTextBlock_451374" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451373" xlink:to="loc_us-gaap_ScheduleOfAssumptionsUsedTableTextBlock_451374" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451370" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_451373" xlink:to="loc_us-gaap_StatementTable_451370" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeAxis" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_547288" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451370" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_547288" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_547289" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_547288" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_547289" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_547289_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_547288" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_547289_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock_451375" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_451373" xlink:to="loc_us-gaap_ScheduleOfCostsOfRetirementPlansTableTextBlock_451375" use="optional" order="6" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SignificantAccountingPoliciesLineItems" xlink:label="loc_utl_SignificantAccountingPoliciesLineItems_452457" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetRetirementObligationsNoncurrent" xlink:label="loc_us-gaap_AssetRetirementObligationsNoncurrent_452464" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_us-gaap_AssetRetirementObligationsNoncurrent_452464" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SignificantAccountingPoliciesTable" xlink:label="loc_utl_SignificantAccountingPoliciesTable_452453" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_utl_SignificantAccountingPoliciesTable_452453" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetAxis" xlink:label="loc_us-gaap_RegulatoryAssetAxis_452466" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_RegulatoryAssetAxis_452466" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_452467" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryAssetAxis_452466" xlink:to="loc_us-gaap_RegulatoryAssetDomain_452467" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_452467_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryAssetAxis_452466" xlink:to="loc_us-gaap_RegulatoryAssetDomain_452467_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_EnvironmentalAndRateCaseCostsAndOtherExpendituresMember" xlink:label="loc_utl_EnvironmentalAndRateCaseCostsAndOtherExpendituresMember_452468" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_452467" xlink:to="loc_utl_EnvironmentalAndRateCaseCostsAndOtherExpendituresMember_452468" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_452503" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_srt_RangeAxis_452503" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_452504" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_452503" xlink:to="loc_srt_RangeMember_452504" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_452504_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_452503" xlink:to="loc_srt_RangeMember_452504_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_452505" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_452504" xlink:to="loc_srt_MaximumMember_452505" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_452458" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_srt_ProductOrServiceAxis_452458" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_452459" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_452458" xlink:to="loc_srt_ProductsAndServicesDomain_452459" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_452459_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_452458" xlink:to="loc_srt_ProductsAndServicesDomain_452459_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodTypeAxis" xlink:label="loc_utl_PeriodTypeAxis_452469" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_utl_PeriodTypeAxis_452469" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodTypeDomain" xlink:label="loc_utl_PeriodTypeDomain_452470" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_PeriodTypeAxis_452469" xlink:to="loc_utl_PeriodTypeDomain_452470" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodTypeDomain" xlink:label="loc_utl_PeriodTypeDomain_452470_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_PeriodTypeAxis_452469" xlink:to="loc_utl_PeriodTypeDomain_452470_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentRedemptionPeriodFiveMember" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodFiveMember_452471" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_PeriodTypeDomain_452470" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodFiveMember_452471" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452454" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452454" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452455" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452454" xlink:to="loc_us-gaap_ClassOfStockDomain_452455" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452455_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452454" xlink:to="loc_us-gaap_ClassOfStockDomain_452455_default" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodAxis" xlink:label="loc_utl_PeriodAxis_547412" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_utl_PeriodAxis_547412" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodDomain" xlink:label="loc_utl_PeriodDomain_547414" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_PeriodAxis_547412" xlink:to="loc_utl_PeriodDomain_547414" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PeriodDomain" xlink:label="loc_utl_PeriodDomain_547414_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_PeriodAxis_547412" xlink:to="loc_utl_PeriodDomain_547414_default" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_452474" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_srt_StatementGeographicalAxis_452474" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_452475" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_452474" xlink:to="loc_srt_SegmentGeographicalDomain_452475" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_452475_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_452474" xlink:to="loc_srt_SegmentGeographicalDomain_452475_default" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NewEnglandMember" xlink:label="loc_utl_NewEnglandMember_452476" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_452475" xlink:to="loc_utl_NewEnglandMember_452476" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_452477" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_AwardTypeAxis_452477" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452478" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_452477" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452478" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452478_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_452477" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452478_default" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DeferredCompensationPlanMember" xlink:label="loc_utl_DeferredCompensationPlanMember_452479" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452478" xlink:to="loc_utl_DeferredCompensationPlanMember_452479" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_453348" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_dei_LegalEntityAxis_453348" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_453349" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_453348" xlink:to="loc_dei_EntityDomain_453349" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_453349_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_453348" xlink:to="loc_dei_EntityDomain_453349_default" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_UtilitiesMember" xlink:label="loc_utl_UtilitiesMember_453351" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_453349" xlink:to="loc_utl_UtilitiesMember_453351" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_OtherSubsidiariesMember" xlink:label="loc_utl_OtherSubsidiariesMember_453353" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_453349" xlink:to="loc_utl_OtherSubsidiariesMember_453353" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FitchburgGasAndElectricLightCompanyMember" xlink:label="loc_utl_FitchburgGasAndElectricLightCompanyMember_1399513" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_453349" xlink:to="loc_utl_FitchburgGasAndElectricLightCompanyMember_1399513" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_547474" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_547474" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_547475" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_547474" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_547475" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_547475_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_547474" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_547475_default" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryAxis" xlink:label="loc_us-gaap_PublicUtilitiesInventoryAxis_571708" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_PublicUtilitiesInventoryAxis_571708" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_571709" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_571708" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_571709" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_571709_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_571708" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_571709_default" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_547277" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_547277" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_547278" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_547277" xlink:to="loc_us-gaap_TypeOfAdoptionMember_547278" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_547278_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_547277" xlink:to="loc_us-gaap_TypeOfAdoptionMember_547278_default" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeAxis" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_547374" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_547374" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_547375" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_547374" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_547375" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_547375_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_547374" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_547375_default" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_1399524" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_1399524" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_1399525" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_1399524" xlink:to="loc_us-gaap_SegmentDomain_1399525" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_1399525_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_1399524" xlink:to="loc_us-gaap_SegmentDomain_1399525_default" use="optional" order="52" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ElectricAndGasDivisionMember" xlink:label="loc_utl_ElectricAndGasDivisionMember_1399528" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_1399525" xlink:to="loc_utl_ElectricAndGasDivisionMember_1399528" use="optional" order="53" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1399517" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1399517" use="optional" order="54" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1399518" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1399517" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1399518" use="optional" order="55" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_1399518_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1399517" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_1399518_default" use="optional" order="56" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_OtherDeferredChargesMember" xlink:label="loc_utl_OtherDeferredChargesMember_1399520" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_1399518" xlink:to="loc_utl_OtherDeferredChargesMember_1399520" use="optional" order="57" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_906710" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_906710" use="optional" order="58" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_906711" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_906710" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_906711" use="optional" order="59" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_906711_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_906710" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_906711_default" use="optional" order="60" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsReceivableMember" xlink:label="loc_us-gaap_AccountsReceivableMember_906712" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_906711" xlink:to="loc_us-gaap_AccountsReceivableMember_906712" use="optional" order="61" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_906713" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_SignificantAccountingPoliciesTable_452453" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_906713" use="optional" order="62" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_906714" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_906713" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_906714" use="optional" order="63" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_906714_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_906713" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_906714_default" use="optional" order="64" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnbilledRevenuesMember" xlink:label="loc_us-gaap_UnbilledRevenuesMember_906715" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_906714" xlink:to="loc_us-gaap_UnbilledRevenuesMember_906715" use="optional" order="65" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_LengthOfPipeline" xlink:label="loc_utl_LengthOfPipeline_452506" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_utl_LengthOfPipeline_452506" use="optional" order="66" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TradingSecurities" xlink:label="loc_us-gaap_TradingSecurities_452463" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_us-gaap_TradingSecurities_452463" use="optional" order="67" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseTermOfContract" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_452502" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_452502" use="optional" order="68" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NumberOfSubsidiaries" xlink:label="loc_utl_NumberOfSubsidiaries_453347" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_utl_NumberOfSubsidiaries_453347" use="optional" order="69" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_CashDeposits" xlink:label="loc_utl_CashDeposits_452473" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_utl_CashDeposits_452473" use="optional" order="70" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_820339" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_820339" use="optional" order="71" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_HardshipAccountsInRegulatoryAssets" xlink:label="loc_utl_HardshipAccountsInRegulatoryAssets_1399510" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_utl_HardshipAccountsInRegulatoryAssets_1399510" use="optional" order="72" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssets" xlink:label="loc_us-gaap_RegulatoryAssets_906720" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SignificantAccountingPoliciesLineItems_452457" xlink:to="loc_us-gaap_RegulatoryAssets_906720" use="optional" order="73" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfGasAndElectricOperatingRevenueDetail">
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_OperatingRevenuesLineItems" xlink:label="loc_utl_OperatingRevenuesLineItems_451771" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451756" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_utl_OperatingRevenuesLineItems_451771" xlink:to="loc_us-gaap_StatementTable_451756" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_451757" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451756" xlink:to="loc_srt_MajorCustomersAxis_451757" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_451758" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_451757" xlink:to="loc_srt_NameOfMajorCustomerDomain_451758" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_451758_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_451757" xlink:to="loc_srt_NameOfMajorCustomerDomain_451758_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ResidentialMember" xlink:label="loc_utl_ResidentialMember_451760" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_451758" xlink:to="loc_utl_ResidentialMember_451760" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_CommercialAndIndustrialMember" xlink:label="loc_utl_CommercialAndIndustrialMember_451761" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_451758" xlink:to="loc_utl_CommercialAndIndustrialMember_451761" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherCustomerMember" xlink:label="loc_us-gaap_OtherCustomerMember_451762" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_451758" xlink:to="loc_us-gaap_OtherCustomerMember_451762" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451767" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451756" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451767" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_451768" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451767" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_451768" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_451768_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451767" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_451768_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_BilledAndUnbilledRevenueMember" xlink:label="loc_utl_BilledAndUnbilledRevenueMember_451769" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_451768" xlink:to="loc_utl_BilledAndUnbilledRevenueMember_451769" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RateAdjustmentMechanismRevenueMember" xlink:label="loc_utl_RateAdjustmentMechanismRevenueMember_451770" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_451768" xlink:to="loc_utl_RateAdjustmentMechanismRevenueMember_451770" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_451763" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451756" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_451763" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_451764" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_451763" xlink:to="loc_us-gaap_SegmentDomain_451764" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_451764_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_451763" xlink:to="loc_us-gaap_SegmentDomain_451764_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GasSegmentMember" xlink:label="loc_utl_GasSegmentMember_451765" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_451764" xlink:to="loc_utl_GasSegmentMember_451765" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ElectricMember" xlink:label="loc_utl_ElectricMember_451766" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_451764" xlink:to="loc_utl_ElectricMember_451766" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_451772" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_OperatingRevenuesLineItems_451771" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_451772" use="optional" order="18" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfAccruedRevenueDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredRevenueArrangementLineItems" xlink:label="loc_us-gaap_DeferredRevenueArrangementLineItems_451791" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetsCurrent" xlink:label="loc_us-gaap_RegulatoryAssetsCurrent_451792" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_451791" xlink:to="loc_us-gaap_RegulatoryAssetsCurrent_451792" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnbilledContractsReceivable" xlink:label="loc_us-gaap_UnbilledContractsReceivable_567150" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_451791" xlink:to="loc_us-gaap_UnbilledContractsReceivable_567150" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredRevenueArrangementByTypeTable" xlink:label="loc_us-gaap_DeferredRevenueArrangementByTypeTable_451784" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DeferredRevenueArrangementLineItems_451791" xlink:to="loc_us-gaap_DeferredRevenueArrangementByTypeTable_451784" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_451785" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DeferredRevenueArrangementByTypeTable_451784" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_451785" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451786" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451785" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451786" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451786_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451785" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451786_default" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RegulatoryAssetMember" xlink:label="loc_utl_RegulatoryAssetMember_451787" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_451786" xlink:to="loc_utl_RegulatoryAssetMember_451787" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451788" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DeferredRevenueArrangementByTypeTable_451784" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451788" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_451789" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451788" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_451789" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_451789_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_451788" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_451789_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnbilledRevenuesMember" xlink:label="loc_us-gaap_UnbilledRevenuesMember_451790" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_451789" xlink:to="loc_us-gaap_UnbilledRevenuesMember_451790" use="optional" order="11" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfExchangeGasReceivableDetail">
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ReceivablesLineItems" xlink:label="loc_utl_ReceivablesLineItems_451800" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ReceivablesTable" xlink:label="loc_utl_ReceivablesTable_451795" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_utl_ReceivablesLineItems_451800" xlink:to="loc_utl_ReceivablesTable_451795" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_451796" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_ReceivablesTable_451795" xlink:to="loc_dei_LegalEntityAxis_451796" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451797" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_451796" xlink:to="loc_dei_EntityDomain_451797" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451797_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_451796" xlink:to="loc_dei_EntityDomain_451797_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NorthernUtilitiesIncMember" xlink:label="loc_utl_NorthernUtilitiesIncMember_451798" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_451797" xlink:to="loc_utl_NorthernUtilitiesIncMember_451798" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FitchburgGasAndElectricLightCompanyMember" xlink:label="loc_utl_FitchburgGasAndElectricLightCompanyMember_451799" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_451797" xlink:to="loc_utl_FitchburgGasAndElectricLightCompanyMember_451799" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherReceivablesNetCurrent" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_451801" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_ReceivablesLineItems_451800" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_451801" use="optional" order="7" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfGasInventoryDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryLineItems" xlink:label="loc_us-gaap_PublicUtilitiesInventoryLineItems_451812" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryTable" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTable_451804" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PublicUtilitiesInventoryLineItems_451812" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTable_451804" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryAxis" xlink:label="loc_us-gaap_PublicUtilitiesInventoryAxis_451805" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTable_451804" xlink:to="loc_us-gaap_PublicUtilitiesInventoryAxis_451805" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_451805" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_451805" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryNaturalGasMember" xlink:label="loc_us-gaap_PublicUtilitiesInventoryNaturalGasMember_451807" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806" xlink:to="loc_us-gaap_PublicUtilitiesInventoryNaturalGasMember_451807" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesInventoryPropaneMember" xlink:label="loc_us-gaap_PublicUtilitiesInventoryPropaneMember_451808" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_451806" xlink:to="loc_us-gaap_PublicUtilitiesInventoryPropaneMember_451808" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ReserveQuantitiesByTypeOfReserveAxis" xlink:label="loc_srt_ReserveQuantitiesByTypeOfReserveAxis_451809" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTable_451804" xlink:to="loc_srt_ReserveQuantitiesByTypeOfReserveAxis_451809" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TypeOfReserveDomain" xlink:label="loc_srt_TypeOfReserveDomain_451810" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ReserveQuantitiesByTypeOfReserveAxis_451809" xlink:to="loc_srt_TypeOfReserveDomain_451810" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_TypeOfReserveDomain" xlink:label="loc_srt_TypeOfReserveDomain_451810_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ReserveQuantitiesByTypeOfReserveAxis_451809" xlink:to="loc_srt_TypeOfReserveDomain_451810_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_NaturalGasLiquidsReservesMember" xlink:label="loc_srt_NaturalGasLiquidsReservesMember_451811" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TypeOfReserveDomain_451810" xlink:to="loc_srt_NaturalGasLiquidsReservesMember_451811" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_WeightedAverageCostInventoryAmount" xlink:label="loc_us-gaap_WeightedAverageCostInventoryAmount_451813" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryLineItems_451812" xlink:to="loc_us-gaap_WeightedAverageCostInventoryAmount_451813" use="optional" order="11" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RegulatoryAssetsDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetLineItems" xlink:label="loc_us-gaap_RegulatoryAssetLineItems_451831" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetsNoncurrent" xlink:label="loc_us-gaap_RegulatoryAssetsNoncurrent_451834" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_451831" xlink:to="loc_us-gaap_RegulatoryAssetsNoncurrent_451834" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssets" xlink:label="loc_us-gaap_RegulatoryAssets_451832" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_451831" xlink:to="loc_us-gaap_RegulatoryAssets_451832" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRegulatoryAssetsTable" xlink:label="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_451831" xlink:to="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetAxis" xlink:label="loc_us-gaap_RegulatoryAssetAxis_451820" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" xlink:to="loc_us-gaap_RegulatoryAssetAxis_451820" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_451821" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryAssetAxis_451820" xlink:to="loc_us-gaap_RegulatoryAssetDomain_451821" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_451821_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryAssetAxis_451820" xlink:to="loc_us-gaap_RegulatoryAssetDomain_451821_default" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451822" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_451821" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451822" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ElectricUtilitiesMember" xlink:label="loc_utl_ElectricUtilitiesMember_451823" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_451821" xlink:to="loc_utl_ElectricUtilitiesMember_451823" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncomeTaxesMember" xlink:label="loc_utl_IncomeTaxesMember_451824" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_451821" xlink:to="loc_utl_IncomeTaxesMember_451824" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_451825" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_451825" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451826" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451825" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451826" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451826_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451825" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451826_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_OtherDeferredChargesMember" xlink:label="loc_utl_OtherDeferredChargesMember_451827" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_451826" xlink:to="loc_utl_OtherDeferredChargesMember_451827" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_451828" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_451828" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="loc_us-gaap_LossContingencyNatureDomain_451829" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_451828" xlink:to="loc_us-gaap_LossContingencyNatureDomain_451829" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="loc_us-gaap_LossContingencyNatureDomain_451829_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_451828" xlink:to="loc_us-gaap_LossContingencyNatureDomain_451829_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_EnvironmentalMattersMember" xlink:label="loc_utl_EnvironmentalMattersMember_451830" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_451829" xlink:to="loc_utl_EnvironmentalMattersMember_451830" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_451817" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_451816" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_451817" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_451818" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_451817" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_451818" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_451818_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_451817" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_451818_default" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ContractualObligationsMember" xlink:label="loc_utl_ContractualObligationsMember_451819" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_451818" xlink:to="loc_utl_ContractualObligationsMember_451819" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetsCurrent" xlink:label="loc_us-gaap_RegulatoryAssetsCurrent_451833" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_451831" xlink:to="loc_us-gaap_RegulatoryAssetsCurrent_451833" use="optional" order="22" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RegulatoryLiabilitiesDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilitiesLineItems" xlink:label="loc_us-gaap_RegulatoryLiabilitiesLineItems_451847" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityNoncurrent" xlink:label="loc_us-gaap_RegulatoryLiabilityNoncurrent_451850" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilitiesLineItems_451847" xlink:to="loc_us-gaap_RegulatoryLiabilityNoncurrent_451850" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityCurrent" xlink:label="loc_us-gaap_RegulatoryLiabilityCurrent_451849" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilitiesLineItems_451847" xlink:to="loc_us-gaap_RegulatoryLiabilityCurrent_451849" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilities" xlink:label="loc_us-gaap_RegulatoryLiabilities_451848" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilitiesLineItems_451847" xlink:to="loc_us-gaap_RegulatoryLiabilities_451848" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRegulatoryLiabilitiesTable" xlink:label="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RegulatoryLiabilitiesLineItems_451847" xlink:to="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_547556" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" xlink:to="loc_dei_LegalEntityAxis_547556" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_547557" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_547556" xlink:to="loc_dei_EntityDomain_547557" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_547557_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_547556" xlink:to="loc_dei_EntityDomain_547557_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_451841" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_451841" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_451842" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_451841" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_451842" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_451842_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_451841" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_451842_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_451844" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_451844" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451845" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451844" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451845" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_451845_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_451844" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_451845_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityAxis" xlink:label="loc_us-gaap_RegulatoryLiabilityAxis_451838" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryLiabilitiesTable_451837" xlink:to="loc_us-gaap_RegulatoryLiabilityAxis_451838" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_451839" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_451838" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_451839" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_451839_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_451838" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_451839_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RateAdjustmentMechanismsMember" xlink:label="loc_utl_RateAdjustmentMechanismsMember_547552" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_451839" xlink:to="loc_utl_RateAdjustmentMechanismsMember_547552" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherRegulatoryAssetsLiabilitiesMember" xlink:label="loc_us-gaap_OtherRegulatoryAssetsLiabilitiesMember_567162" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_451839" xlink:to="loc_us-gaap_OtherRegulatoryAssetsLiabilitiesMember_567162" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncomeTaxRelatedLiabilitiesMember" xlink:label="loc_utl_IncomeTaxRelatedLiabilitiesMember_451840" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_451839" xlink:to="loc_utl_IncomeTaxRelatedLiabilitiesMember_451840" use="optional" order="19" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/FairValueOfMarketableSecuritiesDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_451865" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable" xlink:label="loc_utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable_451853" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_451865" xlink:to="loc_utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable_451853" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_451854" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable_451853" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_451854" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_451855" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_451854" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_451855" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_451855_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_451854" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_451855_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_451856" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_451855" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_451856" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_InvestmentTypesAxis" xlink:label="loc_utl_InvestmentTypesAxis_451857" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable_451853" xlink:to="loc_utl_InvestmentTypesAxis_451857" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_InvestmentsTypeCategorizationDomain" xlink:label="loc_utl_InvestmentsTypeCategorizationDomain_547573" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_InvestmentTypesAxis_451857" xlink:to="loc_utl_InvestmentsTypeCategorizationDomain_547573" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_InvestmentsTypeCategorizationDomain" xlink:label="loc_utl_InvestmentsTypeCategorizationDomain_547573_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_InvestmentTypesAxis_451857" xlink:to="loc_utl_InvestmentsTypeCategorizationDomain_547573_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="loc_us-gaap_MoneyMarketFundsMember_547576" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_InvestmentsTypeCategorizationDomain_547573" xlink:to="loc_us-gaap_MoneyMarketFundsMember_547576" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityFundsMember" xlink:label="loc_us-gaap_EquityFundsMember_547574" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_InvestmentsTypeCategorizationDomain_547573" xlink:to="loc_us-gaap_EquityFundsMember_547574" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_451862" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTablTable_451853" xlink:to="loc_us-gaap_AwardTypeAxis_451862" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451863" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_451862" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451863" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451863_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_451862" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451863_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DeferredCompensationPlanMember" xlink:label="loc_utl_DeferredCompensationPlanMember_451864" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_451863" xlink:to="loc_utl_DeferredCompensationPlanMember_451864" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TradingSecurities" xlink:label="loc_us-gaap_TradingSecurities_451866" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_451865" xlink:to="loc_us-gaap_TradingSecurities_451866" use="optional" order="15" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfEnergySupplyObligationsDetail">
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ContractualObligationLineItems" xlink:label="loc_utl_ContractualObligationLineItems_452495" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnergyMarketingContractLiabilitiesCurrent" xlink:label="loc_us-gaap_EnergyMarketingContractLiabilitiesCurrent_452496" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_ContractualObligationLineItems_452495" xlink:to="loc_us-gaap_EnergyMarketingContractLiabilitiesCurrent_452496" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452483" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_utl_ContractualObligationLineItems_452495" xlink:to="loc_us-gaap_StatementTable_452483" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityAxis" xlink:label="loc_us-gaap_RegulatoryLiabilityAxis_452491" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452483" xlink:to="loc_us-gaap_RegulatoryLiabilityAxis_452491" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_452492" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_452491" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_452492" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_452492_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_452491" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_452492_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DivestitureMember" xlink:label="loc_utl_DivestitureMember_452494" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_452492" xlink:to="loc_utl_DivestitureMember_452494" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RenewableEnergyPortfolioRequirementMember" xlink:label="loc_utl_RenewableEnergyPortfolioRequirementMember_452493" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_452492" xlink:to="loc_utl_RenewableEnergyPortfolioRequirementMember_452493" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_452487" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452483" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_452487" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_452488" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_452487" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_452488" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_452488_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_452487" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_452488_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ObligationsMember" xlink:label="loc_us-gaap_ObligationsMember_452489" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_452488" xlink:to="loc_us-gaap_ObligationsMember_452489" use="optional" order="11" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DividendsDeclaredPerShareDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsPayableLineItems" xlink:label="loc_us-gaap_DividendsPayableLineItems_451885" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsPayableDateDeclaredDayMonthAndYear" xlink:label="loc_us-gaap_DividendsPayableDateDeclaredDayMonthAndYear_451886" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsPayableLineItems_451885" xlink:to="loc_us-gaap_DividendsPayableDateDeclaredDayMonthAndYear_451886" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsPayableDateOfRecordDayMonthAndYear" xlink:label="loc_us-gaap_DividendsPayableDateOfRecordDayMonthAndYear_451888" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsPayableLineItems_451885" xlink:to="loc_us-gaap_DividendsPayableDateOfRecordDayMonthAndYear_451888" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockDividendsPerShareDeclared" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_451889" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsPayableLineItems_451885" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_451889" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsPayableTable" xlink:label="loc_us-gaap_DividendsPayableTable_451875" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DividendsPayableLineItems_451885" xlink:to="loc_us-gaap_DividendsPayableTable_451875" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsAxis" xlink:label="loc_us-gaap_DividendsAxis_451876" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DividendsPayableTable_451875" xlink:to="loc_us-gaap_DividendsAxis_451876" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsDomain" xlink:label="loc_us-gaap_DividendsDomain_451877" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DividendsAxis_451876" xlink:to="loc_us-gaap_DividendsDomain_451877" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsDomain" xlink:label="loc_us-gaap_DividendsDomain_451877_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DividendsAxis_451876" xlink:to="loc_us-gaap_DividendsDomain_451877_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupOneMember" xlink:label="loc_utl_GroupOneMember_451878" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupOneMember_451878" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupTwoMember" xlink:label="loc_utl_GroupTwoMember_451879" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupTwoMember_451879" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupThreeMember" xlink:label="loc_utl_GroupThreeMember_451880" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupThreeMember_451880" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupFiveMember" xlink:label="loc_utl_GroupFiveMember_451882" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupFiveMember_451882" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupSixMember" xlink:label="loc_utl_GroupSixMember_451883" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupSixMember_451883" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupSevenMember" xlink:label="loc_utl_GroupSevenMember_451884" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupSevenMember_451884" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GroupFourMember" xlink:label="loc_utl_GroupFourMember_451881" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_451877" xlink:to="loc_utl_GroupFourMember_451881" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendPayableDateToBePaidDayMonthAndYear" xlink:label="loc_us-gaap_DividendPayableDateToBePaidDayMonthAndYear_451887" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsPayableLineItems_451885" xlink:to="loc_us-gaap_DividendPayableDateToBePaidDayMonthAndYear_451887" use="optional" order="15" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/SignificantSegmentFinancialDataDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems" xlink:label="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatedAndUnregulatedOperatingRevenue" xlink:label="loc_us-gaap_RegulatedAndUnregulatedOperatingRevenue_452186" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" xlink:to="loc_us-gaap_RegulatedAndUnregulatedOperatingRevenue_452186" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_Assets" xlink:label="loc_us-gaap_Assets_452188" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" xlink:to="loc_us-gaap_Assets_452188" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_452189" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_452189" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_452187" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_452187" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452169" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingReconcilingItemForOperatingProfitLossFromSegmentToConsolidatedLineItems_452172" xlink:to="loc_us-gaap_StatementTable_452169" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis" xlink:label="loc_us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis_452182" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452169" xlink:to="loc_us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis_452182" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatedAndUnregulatedOperationDomain" xlink:label="loc_us-gaap_RegulatedAndUnregulatedOperationDomain_452183" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis_452182" xlink:to="loc_us-gaap_RegulatedAndUnregulatedOperationDomain_452183" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatedAndUnregulatedOperationDomain" xlink:label="loc_us-gaap_RegulatedAndUnregulatedOperationDomain_452183_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis_452182" xlink:to="loc_us-gaap_RegulatedAndUnregulatedOperationDomain_452183_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnregulatedOperationMember" xlink:label="loc_us-gaap_UnregulatedOperationMember_452184" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatedAndUnregulatedOperationDomain_452183" xlink:to="loc_us-gaap_UnregulatedOperationMember_452184" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452170" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452169" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452170" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452171" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452170" xlink:to="loc_us-gaap_ClassOfStockDomain_452171" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452171_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452170" xlink:to="loc_us-gaap_ClassOfStockDomain_452171_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_452173" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452169" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_452173" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_452174" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_452173" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_452174" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_452174_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_452173" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_452174_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_BilledAndUnbilledRevenueMember" xlink:label="loc_utl_BilledAndUnbilledRevenueMember_452175" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_452174" xlink:to="loc_utl_BilledAndUnbilledRevenueMember_452175" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RateAdjustmentMechanismRevenueMember" xlink:label="loc_utl_RateAdjustmentMechanismRevenueMember_452176" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_452174" xlink:to="loc_utl_RateAdjustmentMechanismRevenueMember_452176" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_452177" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452169" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_452177" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_452178" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_452177" xlink:to="loc_us-gaap_SegmentDomain_452178" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_452178_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_452177" xlink:to="loc_us-gaap_SegmentDomain_452178_default" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GasSegmentMember" xlink:label="loc_utl_GasSegmentMember_452179" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_452178" xlink:to="loc_utl_GasSegmentMember_452179" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AllOtherSegmentsMember" xlink:label="loc_us-gaap_AllOtherSegmentsMember_452181" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_452178" xlink:to="loc_us-gaap_AllOtherSegmentsMember_452181" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ElectricMember" xlink:label="loc_utl_ElectricMember_452180" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_452178" xlink:to="loc_utl_ElectricMember_452180" use="optional" order="23" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DetailsOnLongTermDebtDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_452374" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_452406" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_LongTermDebt_452406" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtCurrent" xlink:label="loc_us-gaap_LongTermDebtCurrent_452407" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_LongTermDebtCurrent_452407" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_452408" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_452408" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452371" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_StatementTable_452371" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452375" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452371" xlink:to="loc_dei_LegalEntityAxis_452375" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452376" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452375" xlink:to="loc_dei_EntityDomain_452376" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452376_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452375" xlink:to="loc_dei_EntityDomain_452376_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_UnitilEnergySystemsIncMember" xlink:label="loc_utl_UnitilEnergySystemsIncMember_452377" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452376" xlink:to="loc_utl_UnitilEnergySystemsIncMember_452377" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FitchburgGasAndElectricLightCompanyMember" xlink:label="loc_utl_FitchburgGasAndElectricLightCompanyMember_452378" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452376" xlink:to="loc_utl_FitchburgGasAndElectricLightCompanyMember_452378" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NorthernUtilitiesIncMember" xlink:label="loc_utl_NorthernUtilitiesIncMember_452379" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452376" xlink:to="loc_utl_NorthernUtilitiesIncMember_452379" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GraniteStateGasTransmissionIncMember" xlink:label="loc_utl_GraniteStateGasTransmissionIncMember_452380" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452376" xlink:to="loc_utl_GraniteStateGasTransmissionIncMember_452380" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_UnitilRealtyCorpMember" xlink:label="loc_utl_UnitilRealtyCorpMember_906724" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452376" xlink:to="loc_utl_UnitilRealtyCorpMember_906724" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_452381" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452371" xlink:to="loc_us-gaap_DebtInstrumentAxis_452381" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452382" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_452381" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452382" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452382_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_452381" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452382_default" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember" xlink:label="loc_utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember_452391" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember_452391" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember" xlink:label="loc_utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember_452403" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember_452403" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember" xlink:label="loc_utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember_847032" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember_847032" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember" xlink:label="loc_utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember_452400" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember_452400" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember" xlink:label="loc_utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember_452389" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember_452389" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember" xlink:label="loc_utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember_452392" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember_452392" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember" xlink:label="loc_utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember_452395" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember_452395" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember" xlink:label="loc_utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember_452396" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember_452396" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember" xlink:label="loc_utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember_452397" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember_452397" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointSevenEightPercentDueSeptemberFifteenTwentyFourtyMember" xlink:label="loc_utl_SeniorNotesThreePointSevenEightPercentDueSeptemberFifteenTwentyFourtyMember_849604" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesThreePointSevenEightPercentDueSeptemberFifteenTwentyFourtyMember_849604" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointFiveEightPercentDueSeptemberFifteenTwentyFourtyMember" xlink:label="loc_utl_SeniorNotesThreePointFiveEightPercentDueSeptemberFifteenTwentyFourtyMember_849614" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesThreePointFiveEightPercentDueSeptemberFifteenTwentyFourtyMember_849614" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember" xlink:label="loc_utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember_452401" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember_452401" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember" xlink:label="loc_utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember_496007" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember_496007" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember" xlink:label="loc_utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember_566213" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember_566213" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember" xlink:label="loc_utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember_452383" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember_452383" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember" xlink:label="loc_utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember_452394" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember_452394" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember" xlink:label="loc_utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember_452393" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember_452393" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember" xlink:label="loc_utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember_452390" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember_452390" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember" xlink:label="loc_utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember_452388" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember_452388" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember" xlink:label="loc_utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember_452387" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember_452387" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember" xlink:label="loc_utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember_452386" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember_452386" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember" xlink:label="loc_utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember_452384" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452382" xlink:to="loc_utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember_452384" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnamortizedDebtIssuanceExpense" xlink:label="loc_us-gaap_UnamortizedDebtIssuanceExpense_452405" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_UnamortizedDebtIssuanceExpense_452405" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_452404" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452374" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_452404" use="optional" order="39" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DetailsOnLongTermDebtParentheticalDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_452416" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452413" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452416" xlink:to="loc_us-gaap_StatementTable_452413" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452417" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452413" xlink:to="loc_dei_LegalEntityAxis_452417" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452418" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452417" xlink:to="loc_dei_EntityDomain_452418" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452418_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452417" xlink:to="loc_dei_EntityDomain_452418_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_452423" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452413" xlink:to="loc_us-gaap_DebtInstrumentAxis_452423" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452424" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_452423" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452424" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452424_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_452423" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452424_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember" xlink:label="loc_utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember_452439" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember_452439" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember" xlink:label="loc_utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember_452442" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember_452442" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember" xlink:label="loc_utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember_496009" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember_496009" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember" xlink:label="loc_utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember_452428" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsEightPointFourNinePercentSeniorSecuredNotesDueOctoberFourteenTwentyTwentyFourMember_452428" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember" xlink:label="loc_utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember_452425" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember_452425" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember" xlink:label="loc_utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember_452426" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember_452426" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember" xlink:label="loc_utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember_452438" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember_452438" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember" xlink:label="loc_utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember_452429" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember_452429" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember" xlink:label="loc_utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember_566156" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember_566156" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember" xlink:label="loc_utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember_452432" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember_452432" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember" xlink:label="loc_utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember_452430" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember_452430" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsThreePointFiftyEightPercentSeniorSecuredNotesDueSeptemberFifteenTwentyFortyMember" xlink:label="loc_utl_FirstMortgageBondsThreePointFiftyEightPercentSeniorSecuredNotesDueSeptemberFifteenTwentyFortyMember_840101" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsThreePointFiftyEightPercentSeniorSecuredNotesDueSeptemberFifteenTwentyFortyMember_840101" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointSeventyEightPercentDueSeptemberFifteenTwentyFortyMember" xlink:label="loc_utl_SeniorNotesThreePointSeventyEightPercentDueSeptemberFifteenTwentyFortyMember_840139" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesThreePointSeventyEightPercentDueSeptemberFifteenTwentyFortyMember_840139" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember" xlink:label="loc_utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember_452431" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember_452431" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember" xlink:label="loc_utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember_452433" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember_452433" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember" xlink:label="loc_utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember_452434" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember_452434" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember" xlink:label="loc_utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember_452435" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember_452435" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember" xlink:label="loc_utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember_452436" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember_452436" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember" xlink:label="loc_utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember_452437" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember_452437" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember" xlink:label="loc_utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember_840216" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember_840216" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember" xlink:label="loc_utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember_452443" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember_452443" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember" xlink:label="loc_utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember_452445" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452424" xlink:to="loc_utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember_452445" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_452446" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452416" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_452446" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="loc_us-gaap_DebtInstrumentMaturityDate_452447" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452416" xlink:to="loc_us-gaap_DebtInstrumentMaturityDate_452447" use="optional" order="31" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/EstimatedFairValueOfLongTermDebtDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_452235" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452232" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452235" xlink:to="loc_us-gaap_StatementTable_452232" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452233" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452232" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452233" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452234" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452233" xlink:to="loc_us-gaap_ClassOfStockDomain_452234" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452234_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452233" xlink:to="loc_us-gaap_ClassOfStockDomain_452234_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_452236" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452232" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_452236" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_452237" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_452236" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_452237" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_452237_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_452236" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_452237_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_452238" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_452237" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_452238" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LongTermDebtFairValue" xlink:label="loc_us-gaap_LongTermDebtFairValue_452239" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452235" xlink:to="loc_us-gaap_LongTermDebtFairValue_452239" use="optional" order="9" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DebtAndFinancingArrangementsAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_452306" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452303" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_StatementTable_452303" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_452310" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_452310" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_452311" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_452310" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_452311" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_452311_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_452310" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_452311_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AssetsHeldUnderCapitalLeasesMember" xlink:label="loc_us-gaap_AssetsHeldUnderCapitalLeasesMember_452312" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_452311" xlink:to="loc_us-gaap_AssetsHeldUnderCapitalLeasesMember_452312" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452313" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_dei_LegalEntityAxis_452313" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452314" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452313" xlink:to="loc_dei_EntityDomain_452314" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452314_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452313" xlink:to="loc_dei_EntityDomain_452314_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NorthernUtilitiesIncMember" xlink:label="loc_utl_NorthernUtilitiesIncMember_452316" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452314" xlink:to="loc_utl_NorthernUtilitiesIncMember_452316" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_452320" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_us-gaap_DebtInstrumentAxis_452320" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452321" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_452320" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452321" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_452321_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_452320" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_452321_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_CreditFacilityMember" xlink:label="loc_utl_CreditFacilityMember_452322" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_452321" xlink:to="loc_utl_CreditFacilityMember_452322" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_452317" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_us-gaap_CreditFacilityAxis_452317" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_452318" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_452317" xlink:to="loc_us-gaap_CreditFacilityDomain_452318" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_452318_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_452317" xlink:to="loc_us-gaap_CreditFacilityDomain_452318_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_452319" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_452318" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_452319" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_452327" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_us-gaap_TypeOfArrangementAxis_452327" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_452328" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_452327" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_452328" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_452328_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_452327" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_452328_default" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SecondAmendedCreditFacilityMember" xlink:label="loc_utl_SecondAmendedCreditFacilityMember_452329" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_452328" xlink:to="loc_utl_SecondAmendedCreditFacilityMember_452329" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_452323" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_us-gaap_VariableRateAxis_452323" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_452324" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_452323" xlink:to="loc_us-gaap_VariableRateDomain_452324" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_452324_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_452323" xlink:to="loc_us-gaap_VariableRateDomain_452324_default" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_452325" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_452324" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_452325" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_547912" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_srt_RangeAxis_547912" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_547913" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_547912" xlink:to="loc_srt_RangeMember_547913" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_547913_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_547912" xlink:to="loc_srt_RangeMember_547913_default" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="loc_srt_ConsolidatedEntitiesAxis_547905" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452303" xlink:to="loc_srt_ConsolidatedEntitiesAxis_547905" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="loc_srt_ConsolidatedEntitiesDomain_547906" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_547905" xlink:to="loc_srt_ConsolidatedEntitiesDomain_547906" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="loc_srt_ConsolidatedEntitiesDomain_547906_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_547905" xlink:to="loc_srt_ConsolidatedEntitiesDomain_547906_default" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_452333" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_452333" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromLinesOfCredit" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_452336" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_452336" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RepaymentsOfLinesOfCredit" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_452337" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_452337" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_452331" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_452331" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SublimitForIssuanceOfStandbyLettersOfCredit" xlink:label="loc_utl_SublimitForIssuanceOfStandbyLettersOfCredit_452332" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_utl_SublimitForIssuanceOfStandbyLettersOfCredit_452332" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_452357" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_452357" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_452359" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_452359" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_LetterOfCreditFacilityAndBankGuaranteesOutstanding" xlink:label="loc_utl_LetterOfCreditFacilityAndBankGuaranteesOutstanding_452351" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_utl_LetterOfCreditFacilityAndBankGuaranteesOutstanding_452351" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeasePayments" xlink:label="loc_us-gaap_OperatingLeasePayments_452353" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeasePayments_452353" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_452354" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_452354" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_452355" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_452355" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_452356" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_452356" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShortTermDebtWeightedAverageInterestRateOverTime" xlink:label="loc_us-gaap_ShortTermDebtWeightedAverageInterestRateOverTime_463077" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_ShortTermDebtWeightedAverageInterestRateOverTime_463077" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PercentageOfIndebtednessToNetCapital" xlink:label="loc_utl_PercentageOfIndebtednessToNetCapital_547909" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_utl_PercentageOfIndebtednessToNetCapital_547909" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnergyRelatedInventoryNaturalGasInStorage" xlink:label="loc_us-gaap_EnergyRelatedInventoryNaturalGasInStorage_452349" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_EnergyRelatedInventoryNaturalGasInStorage_452349" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaap_AccountsPayableCurrent_452350" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_AccountsPayableCurrent_452350" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_452358" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_452358" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GuaranteeTermPeriod" xlink:label="loc_utl_GuaranteeTermPeriod_906731" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_utl_GuaranteeTermPeriod_906731" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityCurrent" xlink:label="loc_us-gaap_FinanceLeaseLiabilityCurrent_1403911" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_FinanceLeaseLiabilityCurrent_1403911" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityNoncurrent" xlink:label="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_1403912" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_1403912" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseCost" xlink:label="loc_us-gaap_OperatingLeaseCost_1403915" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_452306" xlink:to="loc_us-gaap_OperatingLeaseCost_1403915" use="optional" order="52" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/BorrowingLimitsAmountsOutstandingAndAmountsAvailableUnderRevolvingCreditFacilityDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_452246" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_452250" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452246" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_452250" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShortTermBorrowings" xlink:label="loc_us-gaap_ShortTermBorrowings_452251" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452246" xlink:to="loc_us-gaap_ShortTermBorrowings_452251" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_452252" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452246" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_452252" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452243" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_452246" xlink:to="loc_us-gaap_StatementTable_452243" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452244" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452243" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452244" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452245" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452244" xlink:to="loc_us-gaap_ClassOfStockDomain_452245" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452245_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452244" xlink:to="loc_us-gaap_ClassOfStockDomain_452245_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_452247" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452243" xlink:to="loc_us-gaap_CreditFacilityAxis_452247" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_452248" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_452247" xlink:to="loc_us-gaap_CreditFacilityDomain_452248" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_452248_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_452247" xlink:to="loc_us-gaap_CreditFacilityDomain_452248_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_452249" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_452248" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_452249" use="optional" order="11" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DebtAndFinancingArrangementsClassificationOfTheCompanyLeaseObligationsDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452261" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalLeaseObligationsAbstract" xlink:label="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452261" xlink:to="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_LeaseObligation" xlink:label="loc_utl_LeaseObligation_452270" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" xlink:to="loc_utl_LeaseObligation_452270" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityCurrent" xlink:label="loc_us-gaap_FinanceLeaseLiabilityCurrent_1403916" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" xlink:to="loc_us-gaap_FinanceLeaseLiabilityCurrent_1403916" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityNoncurrent" xlink:label="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_1403917" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" xlink:to="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_1403917" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiability" xlink:label="loc_us-gaap_FinanceLeaseLiability_1403918" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeaseObligationsAbstract_452266" xlink:to="loc_us-gaap_FinanceLeaseLiability_1403918" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452258" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452261" xlink:to="loc_us-gaap_StatementTable_452258" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1403920" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452258" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1403920" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_1403921" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1403920" xlink:to="loc_us-gaap_ClassOfStockDomain_1403921" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_1403921_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1403920" xlink:to="loc_us-gaap_ClassOfStockDomain_1403921_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_LeaseObligationsMember" xlink:label="loc_utl_LeaseObligationsMember_1403922" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_1403921" xlink:to="loc_utl_LeaseObligationsMember_1403922" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract" xlink:label="loc_us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_452262" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452261" xlink:to="loc_us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_452262" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_452263" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_452262" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_452263" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_452264" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_452262" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_452264" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiability" xlink:label="loc_us-gaap_OperatingLeaseLiability_452265" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeaseLiabilitiesPaymentsDueAbstract_452262" xlink:to="loc_us-gaap_OperatingLeaseLiability_452265" use="optional" order="14" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/FutureOperatingLeasePaymentObligationsAndFutureMinimumLeasePaymentsUnderCapitalLeasesDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452277" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract" xlink:label="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452277" xlink:to="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear_452279" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear_452279" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_452281" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo_452281" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_452282" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree_452282" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour_452283" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour_452283" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive_452285" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive_452285" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_452286" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_452286" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ImputedInterestOnOperatingLeases" xlink:label="loc_utl_ImputedInterestOnOperatingLeases_452287" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_utl_ImputedInterestOnOperatingLeases_452287" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OperatingLeaseLiability" xlink:label="loc_us-gaap_OperatingLeaseLiability_452288" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_OperatingLeaseLiability_452288" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths_1403935" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract_452278" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths_1403935" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452274" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452277" xlink:to="loc_us-gaap_StatementTable_452274" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1402494" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452274" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1402494" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_1402495" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1402494" xlink:to="loc_us-gaap_ClassOfStockDomain_1402495" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_1402495_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1402494" xlink:to="loc_us-gaap_ClassOfStockDomain_1402495_default" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_LeaseObligationsMember" xlink:label="loc_utl_LeaseObligationsMember_1402496" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_1402495" xlink:to="loc_utl_LeaseObligationsMember_1402496" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract" xlink:label="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452277" xlink:to="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear_452290" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear_452290" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDueYearTwo" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearTwo_452292" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearTwo_452292" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDueYearThree" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearThree_452293" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearThree_452293" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDueYearFour" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearFour_452294" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueYearFour_452294" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ImputedInterestOnCapitalLeases" xlink:label="loc_utl_ImputedInterestOnCapitalLeases_452298" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_utl_ImputedInterestOnCapitalLeases_452298" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDueAfterYearFive" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueAfterYearFive_452296" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueAfterYearFive_452296" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDue" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDue_452297" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDue_452297" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_AmountOfLeaseObligationFinanceLeaseLiability" xlink:label="loc_utl_AmountOfLeaseObligationFinanceLeaseLiability_452299" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_utl_AmountOfLeaseObligationFinanceLeaseLiability_452299" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_FinanceLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:label="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueNextTwelveMonths_1403936" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalLeasesFutureMinimumPaymentsDueAbstract_452289" xlink:to="loc_us-gaap_FinanceLeaseLiabilityPaymentsDueNextTwelveMonths_1403936" use="optional" order="25" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/CommonStockAndPreferredStockAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_452552" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_452567" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_452567" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452549" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_StatementTable_452549" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_452568" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_us-gaap_AwardTypeAxis_452568" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_452568" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_452568" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_452589" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_452589" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_452570" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452569" xlink:to="loc_us-gaap_RestrictedStockMember_452570" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452550" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452550" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452551" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452550" xlink:to="loc_us-gaap_ClassOfStockDomain_452551" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452551_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452550" xlink:to="loc_us-gaap_ClassOfStockDomain_452551_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_452558" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_452558" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_452559" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_452558" xlink:to="loc_us-gaap_EquityComponentDomain_452559" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_452559_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_452558" xlink:to="loc_us-gaap_EquityComponentDomain_452559_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SeriesSixMember" xlink:label="loc_utl_SeriesSixMember_452561" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_452559" xlink:to="loc_utl_SeriesSixMember_452561" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_452560" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_452559" xlink:to="loc_us-gaap_CommonStockMember_452560" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_452562" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_srt_RangeAxis_452562" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_452563" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_452562" xlink:to="loc_srt_RangeMember_452563" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_452563_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_452562" xlink:to="loc_srt_RangeMember_452563_default" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_452564" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_452563" xlink:to="loc_srt_MaximumMember_452564" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_WeightedAverageMember" xlink:label="loc_srt_WeightedAverageMember_452565" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_452563" xlink:to="loc_srt_WeightedAverageMember_452565" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingAxis" xlink:label="loc_us-gaap_VestingAxis_452574" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_us-gaap_VestingAxis_452574" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_452575" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_452574" xlink:to="loc_us-gaap_VestingDomain_452575" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_452575_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_452574" xlink:to="loc_us-gaap_VestingDomain_452575_default" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_VestingAnnualMember" xlink:label="loc_utl_VestingAnnualMember_452576" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_452575" xlink:to="loc_utl_VestingAnnualMember_452576" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_452555" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_452555" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_452556" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_452555" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_452556" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_452556_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_452555" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_452556_default" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember" xlink:label="loc_utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember_452557" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_452556" xlink:to="loc_utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember_452557" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452593" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452549" xlink:to="loc_dei_LegalEntityAxis_452593" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452594" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452593" xlink:to="loc_dei_EntityDomain_452594" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452594_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452593" xlink:to="loc_dei_EntityDomain_452594_default" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_UnitilEnergySystemsIncMember" xlink:label="loc_utl_UnitilEnergySystemsIncMember_452595" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_452594" xlink:to="loc_utl_UnitilEnergySystemsIncMember_452595" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_452571" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_452571" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_452553" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_452553" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_452554" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_452554" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_452582" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_452582" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledInPeriod" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledInPeriod_452586" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledInPeriod_452586" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCommonSharesWhenSettled" xlink:label="loc_utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCommonSharesWhenSettled_452587" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCommonSharesWhenSettled_452587" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharesIssuedPricePerShare" xlink:label="loc_us-gaap_SharesIssuedPricePerShare_452566" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_SharesIssuedPricePerShare_452566" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCashWhenSettled" xlink:label="loc_utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCashWhenSettled_452588" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCashWhenSettled_452588" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_452572" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_452572" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_452573" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_452573" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockExpense" xlink:label="loc_us-gaap_RestrictedStockExpense_452584" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_RestrictedStockExpense_452584" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_452590" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_452590" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_452585" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_452585" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_452580" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_452580" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_452581" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_452581" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DividendsPreferredStock" xlink:label="loc_us-gaap_DividendsPreferredStock_452600" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_DividendsPreferredStock_452600" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAggregateIntrinsicValue" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAggregateIntrinsicValue_452583" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAggregateIntrinsicValue_452583" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedInPeriodWeightedAverageGrantDateFairValue_452579" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedInPeriodWeightedAverageGrantDateFairValue_452579" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_452577" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_452577" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingNumber" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingNumber_452578" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingNumber_452578" use="optional" order="52" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="loc_us-gaap_PreferredStockDividendRatePercentage_452598" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_PreferredStockDividendRatePercentage_452598" use="optional" order="53" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SharebasedCompensationArrangementBySharebasedPaymentAwardsEquityInstrumentOtherThanOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_utl_SharebasedCompensationArrangementBySharebasedPaymentAwardsEquityInstrumentOtherThanOptionsOutstandingWeightedAverageExercisePrice_452591" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_SharebasedCompensationArrangementBySharebasedPaymentAwardsEquityInstrumentOtherThanOptionsOutstandingWeightedAverageExercisePrice_452591" use="optional" order="54" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FairValueOfRestrictedStockUnitsSettledInCash" xlink:label="loc_utl_FairValueOfRestrictedStockUnitsSettledInCash_452592" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_utl_FairValueOfRestrictedStockUnitsSettledInCash_452592" use="optional" order="55" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_452596" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_452596" use="optional" order="56" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PreferredStockValue" xlink:label="loc_us-gaap_PreferredStockValue_452597" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_452552" xlink:to="loc_us-gaap_PreferredStockValue_452597" use="optional" order="57" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RestrictedStockUnitsIssuedDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_452520" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_452517" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_452520" xlink:to="loc_us-gaap_StatementTable_452517" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_452521" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452517" xlink:to="loc_us-gaap_AwardTypeAxis_452521" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452522" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_452521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452522" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452522_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_452521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452522_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_452523" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_452522" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_452523" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_452518" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_452517" xlink:to="loc_us-gaap_StatementClassOfStockAxis_452518" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452519" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452518" xlink:to="loc_us-gaap_ClassOfStockDomain_452519" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_452519_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_452518" xlink:to="loc_us-gaap_ClassOfStockDomain_452519_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452520" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber" xlink:label="loc_utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber_452530" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" xlink:to="loc_utl_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedNumber_452530" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsDividendEquivalentRightsInPeriod" xlink:label="loc_utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsDividendEquivalentRightsInPeriod_452533" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" xlink:to="loc_utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsDividendEquivalentRightsInPeriod_452533" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettled" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettled_452534" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettled_452534" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_452532" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_452524" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_452532" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_452520" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettledInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettledInPeriodWeightedAverageGrantDateFairValue_452545" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedSettledInPeriodWeightedAverageGrantDateFairValue_452545" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedGrantsInPeriodWeightedAverageGrantDateFairValue_452543" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedGrantsInPeriodWeightedAverageGrantDateFairValue_452543" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedDividendEquivalentRightsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedDividendEquivalentRightsInPeriodWeightedAverageGrantDateFairValue_452544" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" xlink:to="loc_utl_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedDividendEquivalentRightsInPeriodWeightedAverageGrantDateFairValue_452544" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue" xlink:label="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue_452541" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_452535" xlink:to="loc_utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsFullyVestedWeightedAverageGrantDateFairValue_452541" use="optional" order="18" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RegulatoryMattersAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetLineItems" xlink:label="loc_us-gaap_RegulatoryAssetLineItems_547724" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfRegulatoryAssetsTable" xlink:label="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_547732" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_dei_LegalEntityAxis_547732" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_547733" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_547732" xlink:to="loc_dei_EntityDomain_547733" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_547733_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_547732" xlink:to="loc_dei_EntityDomain_547733_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NorthernUtilitiesIncMember" xlink:label="loc_utl_NorthernUtilitiesIncMember_547735" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_547733" xlink:to="loc_utl_NorthernUtilitiesIncMember_547735" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FitchburgGasAndElectricLightCompanyMember" xlink:label="loc_utl_FitchburgGasAndElectricLightCompanyMember_547738" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_547733" xlink:to="loc_utl_FitchburgGasAndElectricLightCompanyMember_547738" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FitchburgGasCompanyMember" xlink:label="loc_utl_FitchburgGasCompanyMember_567188" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_547733" xlink:to="loc_utl_FitchburgGasCompanyMember_567188" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GraniteStateMember" xlink:label="loc_utl_GraniteStateMember_863294" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_547733" xlink:to="loc_utl_GraniteStateMember_863294" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_EnergyAxis" xlink:label="loc_srt_EnergyAxis_547765" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_srt_EnergyAxis_547765" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_EnergyDomain" xlink:label="loc_srt_EnergyDomain_547766" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_EnergyAxis_547765" xlink:to="loc_srt_EnergyDomain_547766" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_EnergyDomain" xlink:label="loc_srt_EnergyDomain_547766_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_EnergyAxis_547765" xlink:to="loc_srt_EnergyDomain_547766_default" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_OffshoreEnergyMember" xlink:label="loc_utl_OffshoreEnergyMember_547768" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EnergyDomain_547766" xlink:to="loc_utl_OffshoreEnergyMember_547768" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_QualifiedCleanEnergyMember" xlink:label="loc_utl_QualifiedCleanEnergyMember_547964" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EnergyDomain_547766" xlink:to="loc_utl_QualifiedCleanEnergyMember_547964" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetAxis" xlink:label="loc_us-gaap_RegulatoryAssetAxis_547769" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_RegulatoryAssetAxis_547769" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_547770" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryAssetAxis_547769" xlink:to="loc_us-gaap_RegulatoryAssetDomain_547770" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryAssetDomain" xlink:label="loc_us-gaap_RegulatoryAssetDomain_547770_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryAssetAxis_547769" xlink:to="loc_us-gaap_RegulatoryAssetDomain_547770_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityAxis" xlink:label="loc_us-gaap_RegulatoryLiabilityAxis_547776" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_RegulatoryLiabilityAxis_547776" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_547777" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_547776" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_547777" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_547777_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_547776" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_547777_default" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_547779" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_547779" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_547780" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_547779" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_547780" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_547780_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_547779" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_547780_default" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherRestructuringMember" xlink:label="loc_us-gaap_OtherRestructuringMember_547781" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_547780" xlink:to="loc_us-gaap_OtherRestructuringMember_547781" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_547756" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_srt_StatementScenarioAxis_547756" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_547758" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_547756" xlink:to="loc_srt_ScenarioUnspecifiedDomain_547758" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_547758_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_547756" xlink:to="loc_srt_ScenarioUnspecifiedDomain_547758_default" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioForecastMember" xlink:label="loc_srt_ScenarioForecastMember_547759" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_547758" xlink:to="loc_srt_ScenarioForecastMember_547759" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_547742" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_srt_StatementGeographicalAxis_547742" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_547743" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_547742" xlink:to="loc_srt_SegmentGeographicalDomain_547743" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_547743_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_547742" xlink:to="loc_srt_SegmentGeographicalDomain_547743_default" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/stpr/2022/stpr-2022.xsd#stpr_ME" xlink:label="loc_stpr_ME_547748" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_547743" xlink:to="loc_stpr_ME_547748" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/stpr/2022/stpr-2022.xsd#stpr_NH" xlink:label="loc_stpr_NH_547755" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_547743" xlink:to="loc_stpr_NH_547755" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_547761" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_srt_RangeAxis_547761" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_547762" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_547761" xlink:to="loc_srt_RangeMember_547762" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_547762_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_547761" xlink:to="loc_srt_RangeMember_547762_default" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_547764" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_547762" xlink:to="loc_srt_MinimumMember_547764" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilityAxis" xlink:label="loc_us-gaap_PublicUtilityAxis_547791" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_PublicUtilityAxis_547791" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilityPlantDomain" xlink:label="loc_us-gaap_UtilityPlantDomain_547792" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilityAxis_547791" xlink:to="loc_us-gaap_UtilityPlantDomain_547792" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UtilityPlantDomain" xlink:label="loc_us-gaap_UtilityPlantDomain_547792_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilityAxis_547791" xlink:to="loc_us-gaap_UtilityPlantDomain_547792_default" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodAxis" xlink:label="loc_us-gaap_TaxPeriodAxis_547782" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_TaxPeriodAxis_547782" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodDomain" xlink:label="loc_us-gaap_TaxPeriodDomain_547783" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_547782" xlink:to="loc_us-gaap_TaxPeriodDomain_547783" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodDomain" xlink:label="loc_us-gaap_TaxPeriodDomain_547783_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_547782" xlink:to="loc_us-gaap_TaxPeriodDomain_547783_default" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_547786" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_VariableRateAxis_547786" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_547787" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_547786" xlink:to="loc_us-gaap_VariableRateDomain_547787" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_547787_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_547786" xlink:to="loc_us-gaap_VariableRateDomain_547787_default" use="optional" order="45" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SecondRateStepAdjustmentsMember" xlink:label="loc_utl_SecondRateStepAdjustmentsMember_547789" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_547787" xlink:to="loc_utl_SecondRateStepAdjustmentsMember_547789" use="optional" order="46" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ProgramAxis" xlink:label="loc_utl_ProgramAxis_1559651" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_utl_ProgramAxis_1559651" use="optional" order="47" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ProgramDomain" xlink:label="loc_utl_ProgramDomain_1559652" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_ProgramAxis_1559651" xlink:to="loc_utl_ProgramDomain_1559652" use="optional" order="48" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ProgramDomain" xlink:label="loc_utl_ProgramDomain_1559652_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_ProgramAxis_1559651" xlink:to="loc_utl_ProgramDomain_1559652_default" use="optional" order="49" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ArrearageManagementProgramMember" xlink:label="loc_utl_ArrearageManagementProgramMember_1559653" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_ProgramDomain_1559652" xlink:to="loc_utl_ArrearageManagementProgramMember_1559653" use="optional" order="50" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_AgreementAxis" xlink:label="loc_utl_AgreementAxis_1559654" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_utl_AgreementAxis_1559654" use="optional" order="51" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_AgreementDomain" xlink:label="loc_utl_AgreementDomain_1559655" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_AgreementAxis_1559654" xlink:to="loc_utl_AgreementDomain_1559655" use="optional" order="52" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_AgreementDomain" xlink:label="loc_utl_AgreementDomain_1559655_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_AgreementAxis_1559654" xlink:to="loc_utl_AgreementDomain_1559655_default" use="optional" order="53" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SettlementAgreementMember" xlink:label="loc_utl_SettlementAgreementMember_1559656" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_AgreementDomain_1559655" xlink:to="loc_utl_SettlementAgreementMember_1559656" use="optional" order="54" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SoliciationAxis" xlink:label="loc_utl_SoliciationAxis_547966" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_utl_SoliciationAxis_547966" use="optional" order="55" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SoliciationDomain" xlink:label="loc_utl_SoliciationDomain_547967" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_utl_SoliciationAxis_547966" xlink:to="loc_utl_SoliciationDomain_547967" use="optional" order="56" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SoliciationDomain" xlink:label="loc_utl_SoliciationDomain_547967_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_utl_SoliciationAxis_547966" xlink:to="loc_utl_SoliciationDomain_547967_default" use="optional" order="57" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_SecondSolicitationMember" xlink:label="loc_utl_SecondSolicitationMember_547969" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SoliciationDomain_547967" xlink:to="loc_utl_SecondSolicitationMember_547969" use="optional" order="58" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FirstSolicitationMember" xlink:label="loc_utl_FirstSolicitationMember_547968" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_SoliciationDomain_547967" xlink:to="loc_utl_FirstSolicitationMember_547968" use="optional" order="59" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_906800" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRegulatoryAssetsTable_547721" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_906800" use="optional" order="60" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_906801" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_906800" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_906801" use="optional" order="61" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_906801_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_906800" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_906801_default" use="optional" order="62" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ContractualObligation" xlink:label="loc_us-gaap_ContractualObligation_547806" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_ContractualObligation_547806" use="optional" order="63" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_FacilityPowerCapacity" xlink:label="loc_utl_FacilityPowerCapacity_547807" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_FacilityPowerCapacity_547807" use="optional" order="64" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RemainingRecoveryPeriodOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided1" xlink:label="loc_us-gaap_RemainingRecoveryPeriodOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided1_547798" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_RemainingRecoveryPeriodOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided1_547798" use="optional" order="65" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue" xlink:label="loc_us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue_547800" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue_547800" use="optional" order="66" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesApprovedDebtCapitalStructurePercentage" xlink:label="loc_us-gaap_PublicUtilitiesApprovedDebtCapitalStructurePercentage_547813" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesApprovedDebtCapitalStructurePercentage_547813" use="optional" order="67" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GeneratingCapacity" xlink:label="loc_utl_GeneratingCapacity_547828" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_GeneratingCapacity_547828" use="optional" order="68" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RemunerationPercentage" xlink:label="loc_utl_RemunerationPercentage_547930" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_RemunerationPercentage_547930" use="optional" order="69" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage" xlink:label="loc_us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage_547811" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage_547811" use="optional" order="70" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage" xlink:label="loc_us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage_547812" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage_547812" use="optional" order="71" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PowerGenerationCapacity" xlink:label="loc_utl_PowerGenerationCapacity_547825" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_PowerGenerationCapacity_547825" use="optional" order="72" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided" xlink:label="loc_us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided_822185" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided_822185" use="optional" order="73" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount" xlink:label="loc_us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount_906718" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount_906718" use="optional" order="74" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage" xlink:label="loc_us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage_906719" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage_906719" use="optional" order="75" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount" xlink:label="loc_us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount_906721" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount_906721" use="optional" order="76" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PublicUtilitiesApprovedRateIncreaseAmount" xlink:label="loc_utl_PublicUtilitiesApprovedRateIncreaseAmount_906722" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_PublicUtilitiesApprovedRateIncreaseAmount_906722" use="optional" order="77" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PublicUtilitiesApprovedRateDecreaseAmount" xlink:label="loc_utl_PublicUtilitiesApprovedRateDecreaseAmount_906723" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_PublicUtilitiesApprovedRateDecreaseAmount_906723" use="optional" order="78" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RevenueImpactThreshold" xlink:label="loc_utl_RevenueImpactThreshold_906725" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_RevenueImpactThreshold_906725" use="optional" order="79" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ThresholdAmountThatWillBeAllowedToAdjustDistributionRatesUpwardOrDownwardDuringTheTermOfStayOutPeriod" xlink:label="loc_utl_ThresholdAmountThatWillBeAllowedToAdjustDistributionRatesUpwardOrDownwardDuringTheTermOfStayOutPeriod_1559657" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_ThresholdAmountThatWillBeAllowedToAdjustDistributionRatesUpwardOrDownwardDuringTheTermOfStayOutPeriod_1559657" use="optional" order="80" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ExpensesAssociatedWithTheProgramExcludedFromTheRevenueRequirementAsPerOrderAndAdjustedIncreaseAmountWillResultInReasonableRates" xlink:label="loc_utl_ExpensesAssociatedWithTheProgramExcludedFromTheRevenueRequirementAsPerOrderAndAdjustedIncreaseAmountWillResultInReasonableRates_1559666" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_ExpensesAssociatedWithTheProgramExcludedFromTheRevenueRequirementAsPerOrderAndAdjustedIncreaseAmountWillResultInReasonableRates_1559666" use="optional" order="81" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PublicUtilitiesApprovedIncreaseAmountOfAnnualRevenueToRecoverEligibleCapitalInvestments" xlink:label="loc_utl_PublicUtilitiesApprovedIncreaseAmountOfAnnualRevenueToRecoverEligibleCapitalInvestments_1559667" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_PublicUtilitiesApprovedIncreaseAmountOfAnnualRevenueToRecoverEligibleCapitalInvestments_1559667" use="optional" order="82" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ApprovedRecoverAmountInCovidNineteenRelatedCostsRelatingToExpensesAsPerAuthorizationProvidedInOrder" xlink:label="loc_utl_ApprovedRecoverAmountInCovidNineteenRelatedCostsRelatingToExpensesAsPerAuthorizationProvidedInOrder_1559668" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetLineItems_547724" xlink:to="loc_utl_ApprovedRecoverAmountInCovidNineteenRelatedCostsRelatingToExpensesAsPerAuthorizationProvidedInOrder_1559668" use="optional" order="83" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/EnvironmentalMattersAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SiteContingencyLineItems" xlink:label="loc_us-gaap_SiteContingencyLineItems_452039" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SiteContingencyTable" xlink:label="loc_us-gaap_SiteContingencyTable_452036" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SiteContingencyLineItems_452039" xlink:to="loc_us-gaap_SiteContingencyTable_452036" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_452041" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SiteContingencyTable_452036" xlink:to="loc_srt_StatementGeographicalAxis_452041" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_452042" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_452041" xlink:to="loc_srt_SegmentGeographicalDomain_452042" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_452042_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_452041" xlink:to="loc_srt_SegmentGeographicalDomain_452042_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NhMember" xlink:label="loc_utl_NhMember_452044" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_452042" xlink:to="loc_utl_NhMember_452044" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_MeMember" xlink:label="loc_utl_MeMember_452043" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_452042" xlink:to="loc_utl_MeMember_452043" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452037" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SiteContingencyTable_452036" xlink:to="loc_dei_LegalEntityAxis_452037" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452038" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452037" xlink:to="loc_dei_EntityDomain_452038" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452038_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452037" xlink:to="loc_dei_EntityDomain_452038_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityAxis" xlink:label="loc_us-gaap_RegulatoryLiabilityAxis_452045" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SiteContingencyTable_452036" xlink:to="loc_us-gaap_RegulatoryLiabilityAxis_452045" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_452046" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_452045" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_452046" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_452046_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_452045" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_452046_default" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnvironmentalRestorationCostsMember" xlink:label="loc_us-gaap_EnvironmentalRestorationCostsMember_452047" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_452046" xlink:to="loc_us-gaap_EnvironmentalRestorationCostsMember_452047" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RemainingAmortizationPeriod" xlink:label="loc_utl_RemainingAmortizationPeriod_452040" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_452039" xlink:to="loc_utl_RemainingAmortizationPeriod_452040" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnvironmentalRemediationExpense" xlink:label="loc_us-gaap_EnvironmentalRemediationExpense_500036" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_452039" xlink:to="loc_us-gaap_EnvironmentalRemediationExpense_500036" use="optional" order="15" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/CompanysLiabilityForEnvironmentalObligationsDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EnvironmentalExitCostLineItems" xlink:label="loc_us-gaap_EnvironmentalExitCostLineItems_452052" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesPayments1" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesPayments1_452057" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesPayments1_452057" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccrualForEnvironmentalLossContingencies" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_452055" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_452055" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses_452056" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses_452056" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccruedEnvironmentalLossContingenciesCurrent" xlink:label="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_452059" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_AccruedEnvironmentalLossContingenciesCurrent_452059" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ScheduleOfEnvironmentalExitCostTable" xlink:label="loc_us-gaap_ScheduleOfEnvironmentalExitCostTable_452049" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_ScheduleOfEnvironmentalExitCostTable_452049" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_452050" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEnvironmentalExitCostTable_452049" xlink:to="loc_dei_LegalEntityAxis_452050" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452051" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_452050" xlink:to="loc_dei_EntityDomain_452051" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_452051_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_452050" xlink:to="loc_dei_EntityDomain_452051_default" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent" xlink:label="loc_us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent_452060" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalExitCostLineItems_452052" xlink:to="loc_us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent_452060" use="optional" order="9" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/DifferencesBetweenProvisionsForIncomeTaxesAndProvisionsCalculatedAtStatutoryFederalTaxRateDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxExaminationLineItems" xlink:label="loc_us-gaap_IncomeTaxExaminationLineItems_547157" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_547158" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_547158" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_547160" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_547160" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_EffectiveIncomeTaxRateReconciliationDomesticIncomeTaxRateDifferential" xlink:label="loc_utl_EffectiveIncomeTaxRateReconciliationDomesticIncomeTaxRateDifferential_547161" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_utl_EffectiveIncomeTaxRateReconciliationDomesticIncomeTaxRateDifferential_547161" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxExaminationTable" xlink:label="loc_us-gaap_IncomeTaxExaminationTable_547154" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_us-gaap_IncomeTaxExaminationTable_547154" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_547170" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_547170" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_567205" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExaminationLineItems_547157" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_567205" use="optional" order="6" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/IncomeTaxesAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncomeTaxesLineItems" xlink:label="loc_utl_IncomeTaxesLineItems_451925" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_TaxCutsAndJobsActOf2017ChangeInTaxRateRegulatoryLiabilityRecoveryRefund" xlink:label="loc_utl_TaxCutsAndJobsActOf2017ChangeInTaxRateRegulatoryLiabilityRecoveryRefund_451953" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_TaxCutsAndJobsActOf2017ChangeInTaxRateRegulatoryLiabilityRecoveryRefund_451953" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_IncomeTaxesTable" xlink:label="loc_utl_IncomeTaxesTable_451924" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_IncomeTaxesTable_451924" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodAxis" xlink:label="loc_us-gaap_TaxPeriodAxis_451930" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_TaxPeriodAxis_451930" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodDomain" xlink:label="loc_us-gaap_TaxPeriodDomain_451931" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_451930" xlink:to="loc_us-gaap_TaxPeriodDomain_451931" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxPeriodDomain" xlink:label="loc_us-gaap_TaxPeriodDomain_451931_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_451930" xlink:to="loc_us-gaap_TaxPeriodDomain_451931_default" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxYear2018Member" xlink:label="loc_us-gaap_TaxYear2018Member_451932" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_451931" xlink:to="loc_us-gaap_TaxYear2018Member_451932" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityAxis" xlink:label="loc_us-gaap_RegulatoryLiabilityAxis_451933" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_RegulatoryLiabilityAxis_451933" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_451934" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_451933" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_451934" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RegulatoryLiabilityDomain" xlink:label="loc_us-gaap_RegulatoryLiabilityDomain_451934_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryLiabilityAxis_451933" xlink:to="loc_us-gaap_RegulatoryLiabilityDomain_451934_default" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_GasRatePayersMember" xlink:label="loc_utl_GasRatePayersMember_451937" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryLiabilityDomain_451934" xlink:to="loc_utl_GasRatePayersMember_451937" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_451938" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_dei_LegalEntityAxis_451938" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451939" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_451938" xlink:to="loc_dei_EntityDomain_451939" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2022/dei-2022.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_451939_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_451938" xlink:to="loc_dei_EntityDomain_451939_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_500032" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_srt_StatementScenarioAxis_500032" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_500033" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_500032" xlink:to="loc_srt_ScenarioUnspecifiedDomain_500033" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_500033_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_500032" xlink:to="loc_srt_ScenarioUnspecifiedDomain_500033_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_451941" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_srt_RangeAxis_451941" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_451942" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_451941" xlink:to="loc_srt_RangeMember_451942" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_451942_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_451941" xlink:to="loc_srt_RangeMember_451942_default" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_451944" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_451942" xlink:to="loc_srt_MinimumMember_451944" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_451943" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_451942" xlink:to="loc_srt_MaximumMember_451943" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_451926" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_451926" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_451927" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_451926" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_451927" use="optional" order="23" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_451927_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_451926" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_451927_default" use="optional" order="24" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxCreditCarryforwardAxis" xlink:label="loc_us-gaap_TaxCreditCarryforwardAxis_451945" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_TaxCreditCarryforwardAxis_451945" use="optional" order="25" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_451946" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_451945" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_451946" use="optional" order="26" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_451946_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_451945" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_451946_default" use="optional" order="27" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityNameAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameAxis_497721" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameAxis_497721" use="optional" order="28" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_497722" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_497721" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_497722" use="optional" order="29" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_IncomeTaxAuthorityNameDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityNameDomain_497722_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityNameAxis_497721" xlink:to="loc_us-gaap_IncomeTaxAuthorityNameDomain_497722_default" use="optional" order="30" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnusualOrInfrequentItemAxis" xlink:label="loc_us-gaap_UnusualOrInfrequentItemAxis_906701" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_us-gaap_UnusualOrInfrequentItemAxis_906701" use="optional" order="31" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnusualOrInfrequentItemDomain" xlink:label="loc_us-gaap_UnusualOrInfrequentItemDomain_906702" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_UnusualOrInfrequentItemAxis_906701" xlink:to="loc_us-gaap_UnusualOrInfrequentItemDomain_906702" use="optional" order="32" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_UnusualOrInfrequentItemDomain" xlink:label="loc_us-gaap_UnusualOrInfrequentItemDomain_906702_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_UnusualOrInfrequentItemAxis_906701" xlink:to="loc_us-gaap_UnusualOrInfrequentItemDomain_906702_default" use="optional" order="33" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_ConsolidatedAppropriationsActTwentyTwentyOneMember" xlink:label="loc_utl_ConsolidatedAppropriationsActTwentyTwentyOneMember_906703" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_UnusualOrInfrequentItemDomain_906702" xlink:to="loc_utl_ConsolidatedAppropriationsActTwentyTwentyOneMember_906703" use="optional" order="34" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_906704" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_utl_IncomeTaxesTable_451924" xlink:to="loc_srt_StatementGeographicalAxis_906704" use="optional" order="35" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_906706" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_906704" xlink:to="loc_srt_SegmentGeographicalDomain_906706" use="optional" order="36" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_906706_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_906704" xlink:to="loc_srt_SegmentGeographicalDomain_906706_default" use="optional" order="37" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_MassachusettsAndMaineMember" xlink:label="loc_utl_MassachusettsAndMaineMember_906707" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_906706" xlink:to="loc_utl_MassachusettsAndMaineMember_906707" use="optional" order="38" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic_451957" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic_451957" use="optional" order="39" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_PercentageOfEmploymentRetentionCredit" xlink:label="loc_utl_PercentageOfEmploymentRetentionCredit_822373" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_PercentageOfEmploymentRetentionCredit_822373" use="optional" order="40" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_EmploymentRetentionDutiesCapacity" xlink:label="loc_utl_EmploymentRetentionDutiesCapacity_822377" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_EmploymentRetentionDutiesCapacity_822377" use="optional" order="41" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_NetOperatingLossCarryforwardsUtilizedForIncomeTaxes" xlink:label="loc_utl_NetOperatingLossCarryforwardsUtilizedForIncomeTaxes_497329" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_NetOperatingLossCarryforwardsUtilizedForIncomeTaxes_497329" use="optional" order="42" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_451949" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_451949" use="optional" order="43" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_RegulatoryLiabilityRecoveryRefund" xlink:label="loc_utl_RegulatoryLiabilityRecoveryRefund_451951" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_RegulatoryLiabilityRecoveryRefund_451951" use="optional" order="44" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_EstimatedFlowBackPeriod" xlink:label="loc_utl_EstimatedFlowBackPeriod_451952" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_utl_IncomeTaxesLineItems_451925" xlink:to="loc_utl_EstimatedFlowBackPeriod_451952" use="optional" order="45" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/KeyWeightedAverageAssumptionsUsedInDeterminingBenefitPlanCostsAndObligationsDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451379" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_StatementTable_451379" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_451380" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451379" xlink:to="loc_us-gaap_StatementClassOfStockAxis_451380" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451381" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451380" xlink:to="loc_us-gaap_ClassOfStockDomain_451381" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451381_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451380" xlink:to="loc_us-gaap_ClassOfStockDomain_451381_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeAxis" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_451383" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451379" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_451383" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451384" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451383" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451384" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451384_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451383" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451384_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_BenefitPlanCostsMember" xlink:label="loc_utl_BenefitPlanCostsMember_451385" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451384" xlink:to="loc_utl_BenefitPlanCostsMember_451385" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_451387" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease_451387" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_451388" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets_451388" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate" xlink:label="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_451386" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate_451386" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear" xlink:label="loc_us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear_451389" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear_451389" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1" xlink:label="loc_us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1_451390" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1_451390" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanYearHealthCareCostTrendRateReachesUltimateTrendRate" xlink:label="loc_us-gaap_DefinedBenefitPlanYearHealthCareCostTrendRateReachesUltimateTrendRate_451391" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451382" xlink:to="loc_us-gaap_DefinedBenefitPlanYearHealthCareCostTrendRateReachesUltimateTrendRate_451391" use="optional" order="14" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/ComponentsOfRetirementPlanCostsDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451403" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_StatementTable_451403" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_451404" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451403" xlink:to="loc_us-gaap_StatementClassOfStockAxis_451404" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451405" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451404" xlink:to="loc_us-gaap_ClassOfStockDomain_451405" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451405_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451404" xlink:to="loc_us-gaap_ClassOfStockDomain_451405_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeAxis" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_451407" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451403" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_451407" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451408" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451407" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451408" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451408_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451407" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451408_default" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PensionPlansDefinedBenefitMember" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_451410" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451408" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_451410" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451413" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451408" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451413" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember" xlink:label="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_451414" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451408" xlink:to="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_451414" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_451420" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_451420" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DefinedBenefitPlanGrossPeriodicBenefitCost" xlink:label="loc_utl_DefinedBenefitPlanGrossPeriodicBenefitCost_451423" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_utl_DefinedBenefitPlanGrossPeriodicBenefitCost_451423" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="utl-20220630.xsd#utl_DefinedBenefitPlanCapitalizedAndDeferred" xlink:label="loc_utl_DefinedBenefitPlanCapitalizedAndDeferred_451424" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_utl_DefinedBenefitPlanCapitalizedAndDeferred_451424" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_451425" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_451425" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanServiceCost" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_451415" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_451415" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanInterestCost" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_451416" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_451416" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_451417" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_451417" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_451419" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451406" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_451419" use="optional" order="18" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://unitil.com/role/RetirementBenefitObligationsAdditionalInformationDetail">
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451454" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_451451" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451454" xlink:to="loc_us-gaap_StatementTable_451451" xbrldt:contextElement="segment" xbrldt:closed="true" use="optional" order="1" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeAxis" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_451455" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451451" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_451455" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451456" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451455" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451456" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_RetirementPlanTypeDomain" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_451456_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_451455" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_451456_default" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451458" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451456" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_451458" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember" xlink:label="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_451459" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451456" xlink:to="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_451459" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PensionPlansDefinedBenefitMember" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_451457" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_451456" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_451457" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_451452" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451451" xlink:to="loc_us-gaap_StatementClassOfStockAxis_451452" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451453" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451452" xlink:to="loc_us-gaap_ClassOfStockDomain_451453" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_451453_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_451452" xlink:to="loc_us-gaap_ClassOfStockDomain_451453_default" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_547607" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451451" xlink:to="loc_srt_StatementScenarioAxis_547607" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_547608" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_547607" xlink:to="loc_srt_ScenarioUnspecifiedDomain_547608" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2022/elts/srt-2022.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_547608_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_547607" xlink:to="loc_srt_ScenarioUnspecifiedDomain_547608_default" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InvestmentTypeAxis" xlink:label="loc_us-gaap_InvestmentTypeAxis_547618" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451451" xlink:to="loc_us-gaap_InvestmentTypeAxis_547618" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_547619" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_547618" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_547619" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_547619_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_547618" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_547619_default" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis" xlink:label="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_547613" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_451451" xlink:to="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_547613" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_547614" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_547613" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_547614" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_PlanAssetCategoriesDomain" xlink:label="loc_us-gaap_PlanAssetCategoriesDomain_547614_default" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis_547613" xlink:to="loc_us-gaap_PlanAssetCategoriesDomain_547614_default" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer" xlink:label="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_547585" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451454" xlink:to="loc_us-gaap_DefinedBenefitPlanContributionsByEmployer_547585" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_906696" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451454" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligationBenefitsPaid_906696" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2022/elts/us-gaap-2022.xsd#us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsRemainderOfFiscalYear" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsRemainderOfFiscalYear_906697" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_451454" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsRemainderOfFiscalYear_906697" use="optional" order="22" />
</definitionLink>
</linkbase>
