<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-10-24</periodOfReport>

    <issuer>
        <issuerCik>0000874691</issuerCik>
        <issuerName>OSI RESTAURANT PARTNERS, INC.</issuerName>
        <issuerTradingSymbol>OSI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001044541</rptOwnerCik>
            <rptOwnerName>WILT TOBY S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OSI RESTAURANT PARTNERS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2202 N. WEST SHORE BLVD., 5TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>TAMPA</rptOwnerCity>
            <rptOwnerState>FL</rptOwnerState>
            <rptOwnerZipCode>33607</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $.01</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>30000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units (&quot;PSU&quot;)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2006-10-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>274.09</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>274.09</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5468.75</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>15.00</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>1998-07-23</value>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2007-07-23</value>
                <footnoteId id="F5"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>45000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>45000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The security converts to common stock on a one-to-one basis.</footnote>
        <footnote id="F2">The PSU were accrued under the Company's Deferred Compensation and Stock Plan (&quot;Plan&quot;) and are settled 100% in the Company's common stock upon cessation as a director.</footnote>
        <footnote id="F3">Not applicable.</footnote>
        <footnote id="F4">Derivative securities beneficially owned at the end of the reported transaction include: (i) 568.70 PSU accrued in 1997 at prices ranging from $15.840 to $17.669; (ii) 416.68 PSU accrued in 1998 at prices ranging from $18.070 to $24.697; (iii) 336.37 PSU accrued in 1999 at prices ranging from $25.33 to $36.68; (iv) 378.49 PSU accrued in 2000 at prices ranging from $23.603 to $29.821; (v) 374.41 PSU accrued in 2001 at prices ranging from $24.85 to $27.98; (vi) 333.57 PSU accrued in 2002 at prices ranging from $28.00 to $37.38; (vii) 345.99 PSU accrued in 2003 at prices ranging from $33.82 to $38.64; (viii) 741.83 PSU accrued in 2004 at prices ranging from $40.38 to $48.36; (ix) 939.1 PSU accrued in 2005 at prices ranging from $39.71 to $46.05; and (x) 740.64 PSU accrued in 2006 at prices ranging from $32.38 to $43.15, all such PSU are to be settled 100% in the Company's common stock upon cessation as a director of the Company.</footnote>
        <footnote id="F5">Currently exercisable. The option vested in three equal annual installments beginning on July 23, 1998.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Joseph J. Kadow, Attorney-in-Fact</signatureName>
        <signatureDate>2006-10-24</signatureDate>
    </ownerSignature>
</ownershipDocument>
