<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2024 Workiva-->
<!--r:10ba7e82-848a-43d1-8115-e850e0fd2171,g:2b37042f-1d78-4afa-91a4-3396538dac92-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link: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"/>
  <link: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"/>
  <link: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"/>
  <link: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"/>
  <link:roleRef roleURI="http://www.bankatfirst.com/role/DocumentandEntityInformationDocument" xlink:type="simple" xlink:href="ffbc-20240514.xsd#DocumentandEntityInformationDocument"/>
  <link:definitionLink xlink:role="http://www.bankatfirst.com/role/DocumentandEntityInformationDocument" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsLineItems_c0eca9a2-82e1-449d-9b5b-6457e055389c" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityListingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsTable_cd417cd9-75a9-4e61-bc6a-beebf5600877" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityListingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityListingsLineItems_c0eca9a2-82e1-449d-9b5b-6457e055389c" xlink:to="loc_dei_EntityListingsTable_cd417cd9-75a9-4e61-bc6a-beebf5600877" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsExchangeAxis_4be203c8-ca81-40b0-8352-36e0dfb8b5b9" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_EntityListingsExchangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntityListingsTable_cd417cd9-75a9-4e61-bc6a-beebf5600877" xlink:to="loc_dei_EntityListingsExchangeAxis_4be203c8-ca81-40b0-8352-36e0dfb8b5b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_4be203c8-ca81-40b0-8352-36e0dfb8b5b9_default" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityListingsExchangeAxis_4be203c8-ca81-40b0-8352-36e0dfb8b5b9" xlink:to="loc_dei_ExchangeDomain_4be203c8-ca81-40b0-8352-36e0dfb8b5b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_334d5d38-8288-4bda-830d-5d19c4251ce9" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityListingsExchangeAxis_4be203c8-ca81-40b0-8352-36e0dfb8b5b9" xlink:to="loc_dei_ExchangeDomain_334d5d38-8288-4bda-830d-5d19c4251ce9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_exch_XNMS_d9bc742e-5cfe-4f7a-a682-4afd94045a8c" xlink:href="https://xbrl.sec.gov/exch/2024/exch-2024.xsd#exch_XNMS"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_334d5d38-8288-4bda-830d-5d19c4251ce9" xlink:to="loc_exch_XNMS_d9bc742e-5cfe-4f7a-a682-4afd94045a8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_9ecc9a97-a666-449e-b9a8-70bf08c7227e" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_c0eca9a2-82e1-449d-9b5b-6457e055389c" xlink:to="loc_dei_Security12bTitle_9ecc9a97-a666-449e-b9a8-70bf08c7227e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_feb321f4-3baf-4472-8b9a-c16ce1cf6124" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_c0eca9a2-82e1-449d-9b5b-6457e055389c" xlink:to="loc_dei_TradingSymbol_feb321f4-3baf-4472-8b9a-c16ce1cf6124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_eb78654d-df0a-494d-b1a7-fc0b8a43c0b2" xlink:href="https://xbrl.sec.gov/dei/2024/dei-2024.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityListingsLineItems_c0eca9a2-82e1-449d-9b5b-6457e055389c" xlink:to="loc_dei_SecurityExchangeName_eb78654d-df0a-494d-b1a7-fc0b8a43c0b2" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
