<SEC-DOCUMENT>0001615219-16-000143.txt : 20160325
<SEC-HEADER>0001615219-16-000143.hdr.sgml : 20160325
<ACCEPTANCE-DATETIME>20160325143010
ACCESSION NUMBER:		0001615219-16-000143
CONFORMED SUBMISSION TYPE:	4/A
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20160322
FILED AS OF DATE:		20160325
DATE AS OF CHANGE:		20160325

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Flex Pharma, Inc.
		CENTRAL INDEX KEY:			0001615219
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				465087339
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		24TH FLOOR
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199
		BUSINESS PHONE:		617-874-1821

	MAIL ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		24TH FLOOR
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Westphal Christoph H
		CENTRAL INDEX KEY:			0001291961

	FILING VALUES:
		FORM TYPE:		4/A
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36812
		FILM NUMBER:		161529467

	MAIL ADDRESS:	
		STREET 1:		C/O ALNYLAM PHARMACEUTICALS, INC
		STREET 2:		300 THIRD STREET
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142
</SEC-HEADER>
<DOCUMENT>
<TYPE>4/A
<SEQUENCE>1
<FILENAME>wf-form4a_145893059937882.xml
<DESCRIPTION>FORM 4/A
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2016-03-22</periodOfReport>

    <dateOfOriginalSubmission>2016-03-23</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001615219</issuerCik>
        <issuerName>Flex Pharma, Inc.</issuerName>
        <issuerTradingSymbol>FLKS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001291961</rptOwnerCik>
            <rptOwnerName>Westphal Christoph H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FLEX PHARMA, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-03-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>23951</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>11.78</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2910955</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See notes</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes></footnotes>

    <remarks>The Form 4 filed on March 23, 2016 reported the purchase of 23,951 shares on March 21, 2016.  This Amendment is being filed to correct the date of purchase. The rest of the information disclosed in the original filing for this purchase, including the footnotes, remains the same.</remarks>

    <ownerSignature>
        <signatureName>/s/ Robert Hadfield, Attorney-in-Fact</signatureName>
        <signatureDate>2016-03-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
