<SEC-DOCUMENT>0001595974-16-000058.txt : 20161117
<SEC-HEADER>0001595974-16-000058.hdr.sgml : 20161117
<ACCEPTANCE-DATETIME>20161117213214
ACCESSION NUMBER:		0001595974-16-000058
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20161115
FILED AS OF DATE:		20161117
DATE AS OF CHANGE:		20161117

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:			RAIFMAN GREGORY R
		CENTRAL INDEX KEY:			0001111909

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36384
		FILM NUMBER:		162006374

	MAIL ADDRESS:	
		STREET 1:		C/O MEDIAPLEX INC
		STREET 2:		177 STEUART STREET 2ND FLOOR
		CITY:			SANFRANCISCO
		STATE:			CA
		ZIP:			94105
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_147943632133626.xml
<DESCRIPTION>FORM 4
<TEXT>
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    <periodOfReport>2016-11-15</periodOfReport>

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

    <reportingOwner>
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            <rptOwnerCik>0001111909</rptOwnerCik>
            <rptOwnerName>RAIFMAN GREGORY R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE RUBICON PROJECT, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>12181 BLUFF CREEK DRIVE, 4TH FLOOR</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</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

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            <transactionDate>
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                <transactionPricePerShare>
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                </transactionShares>
                <transactionPricePerShare>
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            </exerciseDate>
            <expirationDate>
                <value>2023-02-01</value>
            </expirationDate>
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                    <value>Common Stock</value>
                </underlyingSecurityTitle>
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            </securityTitle>
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                <value>7.8</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-05-24</value>
            </transactionDate>
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                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
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            <expirationDate>
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            </expirationDate>
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                </sharesOwnedFollowingTransaction>
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            </securityTitle>
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            </transactionDate>
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                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
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            <transactionTimeliness>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <transactionDate>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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            <exerciseDate>
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            <expirationDate>
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            <underlyingSecurity>
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                    <value>Common Stock</value>
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                <sharesOwnedFollowingTransaction>
                    <value>62585</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
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                    <value>Through the Raifman Family Revocable Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (right to buy)</value>
            </securityTitle>
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                <value>13.16</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-05-24</value>
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                <transactionFormType>5</transactionFormType>
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                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness>
                <value>E</value>
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            <transactionAmounts>
                <transactionShares>
                    <value>58884</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-02-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>58884.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>13.16</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-05-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value>E</value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>58884</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-02-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>58884.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>58884</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Through the Raifman Family Revocable Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On May 24, 2016, the reporting person transferred all of his vested and unvested equity awards to the Raifman Family Revocable Trust. The assignment was for the direct benefit of the reporting person and his family, for estate planning purposes, and does not change beneficial ownership of the shares.</footnote>
        <footnote id="F2">Shares withheld by the issuer to cover the reporting person's tax liability incurred upon the vesting of the reporting person's restricted shares.</footnote>
        <footnote id="F3">The option, representing a right to purchase a total of 785,700 shares, became exercisable with respect to 25% of the grant on January 28, 2014 and becomes exercisable with respect to the remainder in 36 equal installments each calendar month thereafter.</footnote>
        <footnote id="F4">Granted as compensation for services.</footnote>
        <footnote id="F5">25% of the stock options will vest on February 1, 2016 and the remaining option shares will become exercisable in 36 equal installments each calendar month thereafter.</footnote>
        <footnote id="F6">25% of the stock options will vest on February 1, 2017 and the remaining option shares will become exercisable in 36 equal installments each calendar month thereafter.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jonathan Feldman, attorney-in-fact</signatureName>
        <signatureDate>2016-11-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
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</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
