<SEC-DOCUMENT>0001595974-18-000130.txt : 20181206
<SEC-HEADER>0001595974-18-000130.hdr.sgml : 20181206
<ACCEPTANCE-DATETIME>20181206193005
ACCESSION NUMBER:		0001595974-18-000130
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20181203
FILED AS OF DATE:		20181206
DATE AS OF CHANGE:		20181206

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Mandal Sumant
		CENTRAL INDEX KEY:			0001603810

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36384
		FILM NUMBER:		181221976

	MAIL ADDRESS:	
		STREET 1:		12181 BLUFF CREEK DRIVE
		STREET 2:		4TH FLOOR
		CITY:			LOS ANGELES
		STATE:			CA
		ZIP:			90094

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			RUBICON PROJECT, INC.
		CENTRAL INDEX KEY:			0001595974
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-COMPUTER PROGRAMMING, DATA PROCESSING, ETC. [7370]
		IRS NUMBER:				208881738
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		12181 BLUFF CREEK DRIVE, 4TH FLOOR
		CITY:			LOS ANGELES
		STATE:			CA
		ZIP:			90094
		BUSINESS PHONE:		310-207-0272

	MAIL ADDRESS:	
		STREET 1:		12181 BLUFF CREEK DRIVE, 4TH FLOOR
		CITY:			LOS ANGELES
		STATE:			CA
		ZIP:			90094
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_154414258887486.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-12-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001595974</issuerCik>
        <issuerName>RUBICON PROJECT, INC.</issuerName>
        <issuerTradingSymbol>RUBI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001603810</rptOwnerCik>
            <rptOwnerName>Mandal Sumant</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>725 ARIZONA AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 304</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90401</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>41496</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-12-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>508938</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2566856</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects 1,746 shares of Common Stock of the Issuer (the &quot;Shares&quot;) received by the reporting person on December 3, 2018 in connection with pro-rata distributions-in-kind of shares for no consideration.  The aforementioned distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F2">Represents the following distributions on December 3, 2018, for no consideration: (i) Clearstone Venture Partners III-A, L.P., a Delaware limited partnership (&quot;CVP III-A&quot;), distributed 500,000 Shares to its partners, including 1,240 Shares to its General Partner, Clearstone Venture Management III, L.L.C., a Delaware limited liability company (&quot;CVM III&quot;), representing each such partners' pro rata interest in the Shares held by CVP III-A, and (ii) Clearstone Venture Partners III-B, L.P., a Delaware multiple series limited liability company (&quot;CVP III-B&quot;), distributed 8,938 Shares to its members, including the reporting person, representing each such members' pro rata interest in the Shares held by CVP III-B.  On December 3, 2018, CVM III distributed to its members, including the reporting person, the 1,240 Shares that it received as a distribution on December 3, 2018 from CVP III-A.</footnote>
        <footnote id="F3">These distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F4">Represents (i) 11,283 Shares directly beneficially owned by CVM III, (ii) 2,510,692 Shares directly beneficially owned by CVP III-A, and (iii) 44,881 Shares directly beneficially owned by CVP III-B (together with CVM III and CVP III-A, the &quot;Funds&quot;).  The reporting person is a managing member of CVM III and a member of CVP III-B.</footnote>
        <footnote id="F5">The reporting person disclaims beneficial ownership of the Shares held by the Funds, except to the extent of his pecuniary interest therein, if any, and the inclusion of these Shares in this report shall not be deemed an admission of beneficial ownership of the reported shares for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Feldman, attorney-in-fact</signatureName>
        <signatureDate>2018-12-06</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
