<SEC-DOCUMENT>0001595974-17-000024.txt : 20170426
<SEC-HEADER>0001595974-17-000024.hdr.sgml : 20170426
<ACCEPTANCE-DATETIME>20170426212539
ACCESSION NUMBER:		0001595974-17-000024
CONFORMED SUBMISSION TYPE:	4/A
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20170317
FILED AS OF DATE:		20170426
DATE AS OF CHANGE:		20170426

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

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			BARRETT MICHAEL G.
		CENTRAL INDEX KEY:			0001100612

	FILING VALUES:
		FORM TYPE:		4/A
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36384
		FILM NUMBER:		17785981

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		(408) 336-0707

	MAIL ADDRESS:	
		STREET 1:		C/O YAHOO! INC.
		STREET 2:		701 FIRST AVENUE
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089

	FORMER NAME:	
		FORMER CONFORMED NAME:	BARRETT MICHAEL
		DATE OF NAME CHANGE:	19991207
</SEC-HEADER>
<DOCUMENT>
<TYPE>4/A
<SEQUENCE>1
<FILENAME>wf-form4a_149325632252682.xml
<DESCRIPTION>FORM 4/A
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2017-03-17</periodOfReport>

    <dateOfOriginalSubmission>2017-03-22</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001100612</rptOwnerCik>
            <rptOwnerName>BARRETT MICHAEL G.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE RUBICON PROJECT, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>12181 BLUFF CREEK DRIVE, 4TH FL</rptOwnerStreet2>
            <rptOwnerCity>LOS ANGELES</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90094</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-03-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1099138</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1099138</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>5.8</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-03-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>686160</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2027-03-17</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>686160.0</value>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>686160</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Represents restricted stock units that vest as follows: 219,075 on December 21, 2017; 78,608 on May 15, 2018; 137,392 on each November 15 and May 15 thereafter until November 15, 2020; and 114,495 on May 15, 2021.</footnote>
        <footnote id="F2">Granted as compensation for services.</footnote>
        <footnote id="F3">The original Form 4, filed on March 22, 2017, is amended by this Form 4 amendment to correctly reflect the number of stock options granted to the reporting person on March 17, 2017, which were initially reported incorrectly due to a computational error.</footnote>
        <footnote id="F4">25% of the stock options will vest on March 17, 2018 and the remaining option shares will become exercisable in 36 equal installments each calendar month thereafter.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Jonathan Feldman, attorney-in-fact</signatureName>
        <signatureDate>2017-04-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
