Exhibit 24

                               POWER OF ATTORNEY

     Know all by these presents, that the undersigned hereby constitutes and
appoints each of Brent Hatzis-Schoch, Marishka DeToy, Carolyn Ward and Karin
Yoo, signing singly, the undersigned's true and lawful attorney-in-fact to:

     (1)   execute for and on behalf of the undersigned, in the undersigned's
           capacity as an officer and/or director of Black Diamond Therapeutics,
           Inc., a Delaware corporation (the "Company"), (i) Form ID, including
           any attached documents, to effect the assignment of codes to the
           undersigned to be used in the transmission of information to the
           United States Securities and Exchange Commission using the EDGAR
           System, (ii) Forms 3, 4 and 5, (iii) Schedule 13D, (iv) Schedule 13G
           and (v) amendments of each thereof, in accordance with Section 16(a)
           of the Securities Exchange Act of 1934, as amended, 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
           Form 3, 4 or 5, Schedule 13D. Schedule 13G or any amendments thereto
           and timely file such form with the United States Securities and
           Exchange Commission 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 or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's 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 16 or Regulation 13D-G of the Securities Exchange Act of 1934, as
amended. The undersigned hereby agrees to indemnify the attorney-in-fact and the
Company from and against any demand, damage, loss, cost or expense arising from
any false or misleading information provided by the undersigned to the attorney-
in-fact.

     This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file 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.  This Power of Attorney may be filed with the
United States Securities and Exchange Commission as a confirming statement of
the authority granted herein. This Power of Attorney supersedes any prior power
of attorney in connection with the undersigned's capacity as an officer and/or
director of the Company. This Power of Attorney shall expire as to any
individual attorney-in-fact if such attorney-in-fact ceases to be an executive
officer of the Company.

     IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of September 10, 2020.

                                       /s/ Rachel Humphrey
                                       ---------------------------
                                       Rachel Humphrey