Exhibit 24

                               POWER OF ATTORNEY

        With respect to holdings of and transactions in securities issued by
Supernova Partners Acquisition Company III, Ltd. (the "Company"), the
undersigned hereby constitutes and appoints the individuals named on Schedule A
attached hereto and as may be amended from time to time, or any of them signing
singly, with full power of substitution and resubstitution, to act as the
undersigned's true and lawful attorney-in-fact to:

        1.  execute for and on behalf of the undersigned, Schedules 13D and 13G
            in accordance with Section 13 of the Securities Exchange Act of
            1934, as amended (the "Exchange Act"), and the rules thereunder, and
            Forms 3, 4, and 5 in accordance with Section 16 of the Exchange Act
            and the rules thereunder;

        2.  do and perform any and all acts for and on behalf of the undersigned
            which may be necessary or desirable to complete and execute any such
            Schedule 13D or 13G or Form 3, 4, or 5, complete and execute any
            amendment or amendments thereto, and timely file such schedule or
            form with the SEC and any stock exchange or similar authority; and

        3.  take any other action of any type whatsoever in connection with the
            foregoing which, in the opinion of such attorney-in-fact, may be of
            benefit to, in the best interest of, or legally required by, the
            undersigned, it being understood that the documents executed by such
            attorney-in-fact on behalf of the undersigned pursuant to this Power
            of Attorney shall be in such form and shall contain such terms and
            conditions as such attorney-in-fact may approve in such attorney-in-
            fact's discretion.

        The undersigned hereby grants to each such attorney-in-fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the
undersigned might or could do if personally present, with full power of
substitution and resubstitution or revocation, hereby ratifying and confirming
all that such attorney-in-fact, or such attorneys-in-fact substitute or
substitutes, shall lawfully do or cause to be done by virtue of this Power of
Attorney and the rights and powers herein granted.

        The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming,
nor is the Company assuming, any of the undersigned's responsibilities to comply
with Section 13 and Section 16 of the Exchange Act.

        This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Schedule 13D and 13G and Forms 3, 4,
and 5 with respect to the undersigned's holdings of and transactions in
securities issued by the Company, unless earlier revoked by the undersigned in a
signed writing delivered to the foregoing attorneys-in-fact.

        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 25th day of February, 2021.


                                        /s/ James Lanzone
                                        ----------------------------------------
                                              James Lanzone


                                  Schedule A

        Individuals Appointed as Attorney-in-Fact with Full Power of
Substitution and Resubstitution

1. Michael S. Clifton