<SEC-DOCUMENT>0001485003-22-000019.txt : 20220223
<SEC-HEADER>0001485003-22-000019.hdr.sgml : 20220223
<ACCEPTANCE-DATETIME>20220223203420
ACCESSION NUMBER:		0001485003-22-000019
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20220218
FILED AS OF DATE:		20220223
DATE AS OF CHANGE:		20220223

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Forbes Monica
		CENTRAL INDEX KEY:			0001734112

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36296
		FILM NUMBER:		22665999

	MAIL ADDRESS:	
		STREET 1:		3344 NORTH TORREY PINES CT., SUITE 200
		CITY:			SAN DIEGO
		STATE:			CA
		ZIP:			92037

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Sesen Bio, Inc.
		CENTRAL INDEX KEY:			0001485003
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				262025616
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		245 FIRST STREET
		STREET 2:		SUITE 1800
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142
		BUSINESS PHONE:		617-444-8550

	MAIL ADDRESS:	
		STREET 1:		245 FIRST STREET
		STREET 2:		SUITE 1800
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	Eleven Biotherapeutics, Inc.
		DATE OF NAME CHANGE:	20100223
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_164566642051663.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-02-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001485003</issuerCik>
        <issuerName>Sesen Bio, Inc.</issuerName>
        <issuerTradingSymbol>SESN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001734112</rptOwnerCik>
            <rptOwnerName>Forbes Monica</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SESEN BIO, INC</rptOwnerStreet1>
            <rptOwnerStreet2>245 FIRST STREET, SUITE 1800</rptOwnerStreet2>
            <rptOwnerCity>CAMBRIDGE</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02142</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-02-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>542250</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>542250</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents restricted stock units (&quot;RSUs&quot;) which shall vest annually in equal amounts over a four-year period following the grant date, so long as the reporting person remains employed by Sesen Bio, Inc. (the &quot;Company&quot;). Each RSU represents a contingent right to receive one share of the Company's common stock. The compensation committee of the Company approved the grant to the reporting person, based on market analysis by Radford of public, pre-commercial and commercial biopharmaceutical companies.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Mark R. Sullivan, as attorney-in-fact</signatureName>
        <signatureDate>2022-02-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
