<SEC-DOCUMENT>0001537435-20-000098.txt : 20200721
<SEC-HEADER>0001537435-20-000098.hdr.sgml : 20200721
<ACCEPTANCE-DATETIME>20200721121855
ACCESSION NUMBER:		0001537435-20-000098
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20200709
FILED AS OF DATE:		20200721
DATE AS OF CHANGE:		20200721

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			HOLUBOW FRED
		CENTRAL INDEX KEY:			0000905478

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36103
		FILM NUMBER:		201038270

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			TECOGEN INC.
		CENTRAL INDEX KEY:			0001537435
		STANDARD INDUSTRIAL CLASSIFICATION:	AIR COND & WARM AIR HEATING EQUIP & COMM & INDL REFRIG EQUIP [3585]
		IRS NUMBER:				043536131
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		45 FIRST AVENUE
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		781-466-6400

	MAIL ADDRESS:	
		STREET 1:		45 FIRST AVENUE
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	TECOGEN INC
		DATE OF NAME CHANGE:	20111215
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_159534831886004.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-07-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001537435</issuerCik>
        <issuerName>TECOGEN INC.</issuerName>
        <issuerTradingSymbol>TGEN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000905478</rptOwnerCik>
            <rptOwnerName>HOLUBOW FRED</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>45 FIRST AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0.71</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2020-07-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>100000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.71</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2021-07-09</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2030-07-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>100000.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>100000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">25% of options vest on each of the first four anniversaries of the grant date</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Fred Holubow</signatureName>
        <signatureDate>2020-07-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
