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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-12-14</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000034563</issuerCik>
        <issuerName>FARMER BROTHERS CO</issuerName>
        <issuerTradingSymbol>FARM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001314571</rptOwnerCik>
            <rptOwnerName>Waite Carol Farmer</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>FARMER BROS CO</rptOwnerStreet1>
            <rptOwnerStreet2>20333 S NORMANDIE AV</rptOwnerStreet2>
            <rptOwnerCity>TORRANCE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90502</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>trustee, beneficiary, settlor</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001315366</rptOwnerCik>
            <rptOwnerName>Grossman Jeanne Farmer</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>FARMER BROS CO</rptOwnerStreet1>
            <rptOwnerStreet2>20333 S NORMANDIE AV</rptOwnerStreet2>
            <rptOwnerCity>TORRANCE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90502</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>trustee, beneficiary, settlor</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001314759</rptOwnerCik>
            <rptOwnerName>Farmer Richard F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>FARMER BROS CO</rptOwnerStreet1>
            <rptOwnerStreet2>20333 S NORMANDIE AV</rptOwnerStreet2>
            <rptOwnerCity>TORRANCE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>90502</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <otherText>trustee, beneficiary, settlor</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Farmer Bros Co Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-12-14</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2005-12-14</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2620024</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.33</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6398781</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>see footnote 3</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Farmer Bros Co Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-12-14</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2005-12-14</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2620024</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.33</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6398781</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>see footnote 4</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On December 14, 2005 a total of 2,620,024 shares of Common Stock previously reported on Form 3/A filed with the SEC on March 14, 2005 (the &quot;Form 3/A&quot;) were contributed (the &quot;Partnership Contribution&quot;) by various trusts for the benefit of the Farmer family to Farmer Equities, LP, a California limited partnership (the &quot;Partnership&quot;), the partners of which are trusts for the benefit of the Farmer Family.  No shares were purchased or sold.  All share numbers have been rounded to the nearest whole share.  See the Form 3 filed by the Partnership as of the date hereof.</footnote>
        <footnote id="F2">The 6,398,781 shares beneficially owned by all Reporting Persons following the Partnership Contribution include: (i) 6,395,232 shares previously reported on Form 3/A; (ii) 1,055 shares allocated to Roy E. Farmer under the Farmer Bros. Co. Employee Stock Ownership Plan (&quot;ESOP&quot;); and (iii) 2,494 shares distributed by the ESOP for the benefit of Roy F. Farmer.</footnote>
        <footnote id="F3">The 2,620,024 shares contributed to the Partnership include: (i) 1,771,394 shares previously reported as beneficially owned by Ms. Waite and Ms. Grossman as successor co-trustees of various family trusts; and (ii) 848,630 shares previously reported as beneficially owned by Ms. Waite, Ms. Grossman and Dr. Farmer as sucessor co-trustees of various family trusts.</footnote>
        <footnote id="F4">As a result of the Partnership Contribution, the 2,620,024 shares contributed to the Partnership are beneficially owned by Ms. Waite, Ms. Grossman and Dr. Farmer each as a trustee of a trust that is a general partner of the Partnership to the extent of their proportionate interest in the Partnership.  At the option of the Reporting Persons, the entire amount of the Partnership's interest int he shares is being reported.  Each Reporting Person disclaims beneficial ownership of these shares except to the extent of his or her pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Carol Lynn Farmer Waite</signatureName>
        <signatureDate>2005-12-14</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carol Lynn Farmer Waite, Attorney-in Fact for Jeanne Farmer Grossman</signatureName>
        <signatureDate>2005-12-14</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Carol Lynn Farmer Waite, Attorney-in-Fact for Richard F. Farmer</signatureName>
        <signatureDate>2005-12-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
