<SEC-DOCUMENT>0001485003-23-000060.txt : 20230223
<SEC-HEADER>0001485003-23-000060.hdr.sgml : 20230223
<ACCEPTANCE-DATETIME>20230223193338
ACCESSION NUMBER:		0001485003-23-000060
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20230221
FILED AS OF DATE:		20230223
DATE AS OF CHANGE:		20230223

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:		23661774

	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_167719877599557.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2023-02-21</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>2023-02-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>61862</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.5873</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>480388</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The sale reported on this Form 4 represents shares sold by the Reporting Person to cover tax withholding obligations in connection with the vesting and settlement of shares of common stock pursuant to a restricted stock unit award granted on February 18, 2022. The sale is mandated by the Issuer's election to require the satisfaction of tax withholding obligations to be funded by a &quot;sell to cover&quot; transaction and does not represent a discretionary transaction by the Reporting Person.</footnote>
        <footnote id="F2">The Reporting Person hereby undertakes to provide, upon request, to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, full information regarding the number of shares sold at the price set forth above.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Mark R. Sullivan, as attorney-in-fact</signatureName>
        <signatureDate>2023-02-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
