<SEC-DOCUMENT>0001485003-21-000096.txt : 20210505
<SEC-HEADER>0001485003-21-000096.hdr.sgml : 20210505
<ACCEPTANCE-DATETIME>20210505182234
ACCESSION NUMBER:		0001485003-21-000096
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20210503
FILED AS OF DATE:		20210505
DATE AS OF CHANGE:		20210505

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Keyes Jason A
		CENTRAL INDEX KEY:			0001678152

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36296
		FILM NUMBER:		21895012

	MAIL ADDRESS:	
		STREET 1:		3344 NORTH TORREY PINES COURT
		STREET 2:		SUITE 200
		CITY:			LA JOLLA
		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_162025333944120.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-05-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001678152</rptOwnerCik>
            <rptOwnerName>Keyes Jason A</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>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</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.79</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2021-05-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>92500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2031-05-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>92500.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>92500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The option was granted on May 3, 2021 and vests over one year, with 1/12th of the shares subject to the option vesting at the end of each successive one-month period following the grant date until the earlier of a) the day that is one business day prior to the date of the next annual meeting and b) the first anniversary of the grant date, at which time such option shall be fully vested. In April 2021, Sesen Bio, Inc.'s compensation committee, based on market analysis by Radford, recommended to the Sesen Bio, Inc. board of directors, and the board of directors approved, an increase in non-employee director equity compensation from the 50th percentile to the 75th percentile of the Radford market analysis of public, pre-commercial biopharmaceutical companies with less than 200 employees and market values between $250 million and $1 billion. The option granted to purchase 92,500 shares of Sesen Bio, Inc.'s common stock was equivalent to the 75th percentile of such Radford market analysis.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Monica Forbes, as attorney-in-fact</signatureName>
        <signatureDate>2021-05-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
