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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-09-08</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001651052</issuerCik>
        <issuerName>Focus Financial Partners Inc.</issuerName>
        <issuerTradingSymbol>FOCS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001746246</rptOwnerCik>
            <rptOwnerName>Kodialam Rajini Sundar</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>875 THIRD AVENUE, 28TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>COO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
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            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-09-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>260749</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
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                    <value>260749</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
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            <transactionDate>
                <value>2020-09-08</value>
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            <transactionTimeliness>
                <value></value>
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            <transactionAmounts>
                <transactionShares>
                    <value>87574</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
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                </transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
                    <value>87574</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Kodialam 2014 Family Trust</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-09-08</value>
            </transactionDate>
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                <value></value>
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            <transactionAmounts>
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                    <value>260749</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>31.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2020-09-08</value>
            </transactionDate>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>87574</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>31.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Kodialam 2014 Family Trust</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Incentive Units in Focus Financial Partners, LLC</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>9.00</value>
                <footnoteId id="F2"/>
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            <transactionDate>
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            </transactionDate>
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            <transactionTimeliness>
                <value></value>
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            <transactionAmounts>
                <transactionShares>
                    <value>345348</value>
                </transactionShares>
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                </transactionPricePerShare>
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            <expirationDate>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Units in Focus Financial Partners, LLC</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>260749</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>Incentive Units in Focus Financial Partners, LLC</value>
            </securityTitle>
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            <transactionDate>
                <value>2020-09-08</value>
            </transactionDate>
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                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>125000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <expirationDate>
                <footnoteId id="F3"/>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Units in Focus Financial Partners, LLC</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>87574</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>215000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Kodialam 2014 Family Trust</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Common Units in Focus Financial Partners, LLC</value>
            </securityTitle>
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            <transactionDate>
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                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value></value>
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                    <value>260749</value>
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                <transactionPricePerShare>
                    <footnoteId id="F1"/>
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                </transactionAcquiredDisposedCode>
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                <footnoteId id="F1"/>
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            <expirationDate>
                <footnoteId id="F1"/>
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            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>260749</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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                <sharesOwnedFollowingTransaction>
                    <value>260749</value>
                </sharesOwnedFollowingTransaction>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Common Units in Focus Financial Partners, LLC</value>
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                    <value>87574</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
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            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>87574</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>87574</value>
                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Kodialam 2014 Family Trust</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Common Units in Focus Financial Partners, LLC</value>
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            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
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                <value></value>
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                    <value>260749</value>
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                <transactionPricePerShare>
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                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
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            <expirationDate>
                <footnoteId id="F1"/>
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            <underlyingSecurity>
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                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>260749</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
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                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
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        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Common Units in Focus Financial Partners, LLC</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2020-09-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>87574</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>87574</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Kodialam 2014 Family Trust</value>
                    <footnoteId id="F4"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On September 8, 2020 (the &quot;Exchange Date&quot;), pursuant to the Fourth Amended and Restated Operating Agreement of Focus Financial Partners, LLC, as amended (the &quot;Fourth Amended and Restated Focus LLC Agreement&quot;), an aggregate of 470,348 of the reporting persons' vested incentive units in Focus Financial Partners, LLC (&quot;Focus LLC&quot;) were exchanged for an aggregate of 348,323 shares of the Issuer's Class A common stock. Such incentive units were first converted into a number of common units in Focus LLC that took into account the value of the Issuer's Class A common stock, as calculated pursuant to the Fourth Amended and Restated Focus LLC Agreement, and such incentive units' aggregate hurdle amount, and the resulting common units were then exchanged for an equal number of shares of the Issuer's Class A common stock.</footnote>
        <footnote id="F2">Each incentive unit in Focus LLC entitles the holder to receive distributions from Focus LLC if the aggregate distributions made by Focus LLC in respect of each common unit issued and outstanding on or prior to date of the grant of the incentive unit exceeds a specified amount, referred to as the hurdle amount. The hurdle amount is set at the time of grant and typically represents the estimated fair value of a common unit in Focus LLC on the date of grant. The figure reflected in column 2 is the hurdle amount assigned to each incentive award.</footnote>
        <footnote id="F3">Vested incentive units are exchangeable, subject to certain restrictions in the Fourth Amended and Restated Focus LLC Agreement, for (i) a number of shares of the Issuer's Class A common stock that takes into account the value of the Issuer's Class A common stock, as calculated pursuant to the Fourth Amended and Restated Focus LLC Agreement, and such incentive units' aggregate hurdle amount or, (ii) at the election of the Issuer, cash. Upon exchange, such incentive units are first converted into a number of common units in Focus LLC that takes into account the value of the Issuer's Class A common stock, as calculated pursuant to the Fourth Amended and Restated Focus LLC Agreement, and such incentive units' aggregate hurdle amount, and the resulting common units are then exchanged for an equal number of shares of the Issuer's Class A common stock. Incentive units do not expire.</footnote>
        <footnote id="F4">Represents securities held by the Kodialam 2014 Family Trust, an irrevocable grantor trust established by the reporting person for the benefit of her children. The reporting person disclaims beneficial ownership of the securities owned by the trust except to the extent of her indirect pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ J. Russell McGranahan as Attorney-in-Fact</signatureName>
        <signatureDate>2020-09-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
