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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2015-11-27</periodOfReport>

    <dateOfOriginalSubmission>2015-12-01</dateOfOriginalSubmission>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001644378</issuerCik>
        <issuerName>RMR Group Inc.</issuerName>
        <issuerTradingSymbol>NASDAQ:RMR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001456772</rptOwnerCik>
            <rptOwnerName>Government Properties Income Trust</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO NEWTON PLACE</rptOwnerStreet1>
            <rptOwnerStreet2>255 WASHINGTON STREET, SUITE 300</rptOwnerStreet2>
            <rptOwnerCity>NEWTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02458</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, par value $0.001</value>
            </securityTitle>
            <transactionDate>
                <value>2015-11-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>768032</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>773169</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock, par value $0.001</value>
            </securityTitle>
            <transactionDate>
                <value>2015-11-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>441056</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1214225</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On November 13, 2015, the Reporting Person approved the distribution of shares of Class A Common Stock of The RMR Group Inc. (&quot;INC Shares&quot;) pro rata to holders of the Reporting Person's common shares of beneficial interest of record as of November 27, 2015 (the &quot;Distribution&quot;).  The Distribution was paid on December 14, 2015.</footnote>
        <footnote id="F2">This amendment is being filed to revise the number of INC Shares distributed by the Reporting Person following adjustment subsequent to the initial report for required payments to the Depositary Trust Company (&quot;DTC&quot;) of cash in lieu of fractional shares to DTC participants.</footnote>
        <footnote id="F3">On November 13, 2015, Select Income REIT (&quot;SIR&quot;) approved the distribution of INC Shares pro rata to holders of SIR's common shares of beneficial interest of record as of November 27, 2015 (the &quot;SIR Distribution&quot;), and, as a holder of 24,918,421 SIR common shares, the Reporting Person received 441,056 INC Shares from SIR in the SIR Distribution.  The SIR Distribution was paid on December 14, 2015.  The Reporting Person did not pay any consideration for the INC Shares it received in the SIR Distribution.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Mark L. Kleifges, Treasurer and Chief Financial Officer, Government Properties Income Trust</signatureName>
        <signatureDate>2015-12-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
