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

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2011-04-29</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001463258</issuerCik>
        <issuerName>Renewable Energy Group, Inc.</issuerName>
        <issuerTradingSymbol>NONE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001519115</rptOwnerCik>
            <rptOwnerName>USRG Power &amp; Biofuels Fund II GP, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2425 OLYMPIC BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4050W</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001402011</rptOwnerCik>
            <rptOwnerName>USRG POWER &amp; BIOFUELS FUND II LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2425 OLYMPIC BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4050W</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001519093</rptOwnerCik>
            <rptOwnerName>USRG Management Company, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2425 OLYMPIC BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4050W</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001402003</rptOwnerCik>
            <rptOwnerName>USRG POWER &amp; BIOFUELS FUND II-A LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2425 OLYMPIC BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4050W</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001477461</rptOwnerCik>
            <rptOwnerName>Koch Jonathan</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O US RENEWABLES GROUP</rptOwnerStreet1>
            <rptOwnerStreet2>10 BANK STREET, SUITE 580</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001519110</rptOwnerCik>
            <rptOwnerName>USRG Holdco V, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2425 OLYMPIC BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 4050W</rptOwnerStreet2>
            <rptOwnerCity>SANTA MONICA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90404</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $0.0001</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2563268</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock, par value $0.0001</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>11.00</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock, par value $0.0001</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3388635</value>
                    <footnoteId id="F5"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Purchase Warrants (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.25</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-02-26</value>
            </exerciseDate>
            <expirationDate>
                <value>2018-06-25</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock, par value $0.0001</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>190040</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F7"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock, par value $0.0001</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>10298</value>
                    <footnoteId id="F7"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects securities of the issuer owned directly by USRG Holdco , LLC (&quot;Holdco V&quot;).</footnote>
        <footnote id="F2">USRG Power &amp; Biofuels Fund II, LP (&quot;Fund II&quot;) and USRG Power &amp; Biofuels Fund II-A, LP (&quot;Fund II-A&quot;) are members of Holdco V.  USRG Power &amp; Biofuels Fund II GP, LLC (&quot;Fund II GP&quot;) is the general partner of Fund II and Fund II-A.  USRG Management Company, LLC (&quot;Management Company&quot;) is a manager of Holdco V and the management company for Fund II and Fund II-A.  Jonathan Koch is a director of the issuer.  An affiliate of Mr. Koch is a member of Fund II GP and indirectly a member of Management Company.</footnote>
        <footnote id="F3">Fund II, Fund II-A GP, Management Company and Mr. Koch may be deemed to indirectly beneficially own the securities of the issuer owned by Holdco V.  Each of Fund II, Fund II-A, Fund II GP, Management Company and Mr. Koch expressly disclaims beneficial ownership of such securities for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purposes, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F4">The Series A Convertible Preferred Stock (the &quot;Preferred Stock&quot;) is convertible at any time at the holder's election and has no expiration date.  In addition, the Preferred Stock will be converted automatically (i) upon issuer's qualified public offering, (ii) upon the election of holders of 75% of the outstanding Preferred Stock or (iii) if the shares of Common Stock have a closing price on Nasdaq or a national securities exchange in excess of $24.75 per share for ninety (90) consecutive trading days with an average daily trading volume on such trading days of at least $8,000,000.</footnote>
        <footnote id="F5">The Preferred Stock is convertible into a number of shares of Common Stock equal to (i) the original issuance price of the Preferred Stock plus, if the issuer and the holder elect, the accrued and unpaid dividends on the Preferred Stock, divided by (ii) the applicable conversion price on the conversion date.  As of the date of this Form 3, Holdco V owns 3,388,635 shares of Preferred Stock, which are convertible into 3,388,635 shares of Common Stock at a conversion price of $11.00 per share.</footnote>
        <footnote id="F6">The Restricted Stock Units will automatically convert into shares of Common Stock upon vesting.  As of the date of this Form 3, all of the Restricted Stock Units are unvested.  The Restricted Stock Units will vest in full on the earliest of (i) August 18, 2013, (ii) a change of control of the issuer, (iii) the first date that Holdco V may sell Common Stock on Nasdaq or a national securities exchange and (iv) the first three month period following the development of a regular trading market with at least one market maker where trading volume during such three month period is greater than 1% of the outstanding shares of Common Stock.</footnote>
        <footnote id="F7">Holdco V owns 10,298 Restricted Stock Units.  Upon vesting, each Restricted Stock Unit will automatically convert into one share of Common Stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>USRG Holdco V, LLC, By:  USRG Management Company, LLC, its Manager, By /s/ Jonathan Koch, Managing Director</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>USRG Power &amp; Biofuels Fund II, LP, By:  USRG Power &amp; Biofuels Fund II GP, LLC, its General Partner, By /s/ Jonathan Koch, Managing Director</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>USRG Power &amp; Biofuels Fund II-A, LP, By:  USRG Power &amp; Biofuels Fund II GP, LLC, its General Partner, By /s/ Jonathan Koch, Managing Director</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>USRG Power &amp; Biofuels Fund II GP, LLC, By /s/ Jonathan Koch, Managing Director</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>USRG Management Company, LLC, By /s/ Jonathan Koch, Managing Director</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Jonathan Koch, Jonathan Koch</signatureName>
        <signatureDate>2011-04-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
