Exhibit 24

                               POWER OF ATTORNEY

Know all by these present, that the undersigned hereby constitutes and appoints
Angelos Stergiou of SELLAS Life Sciences Group, Inc. (the "Company"), and Daniel
Bagliebter, Cliff Silverman and Daniel Bosworth of Mintz, Levin, Cohn, Ferris,
Glovsky and Popeo, P.C., signing singly, with full power of substitution, the
undersigned's true and lawful attorney-in-fact, to:

     1)  execute and deliver for and on behalf of the undersigned, forms and
         authentication documents for EDGAR Filing Access;

     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
         forms and authentication documents;

     3)  execute and deliver for and on behalf of the undersigned, in the
         undersigned's capacity as an officer, director and/or 10% shareholder
         of the Company, Forms 3, 4 and 5 in accordance with Section 16(a) of
         the Securities Exchange Act of 1934 and the rules thereunder;

     4)  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 and timely file such form with the United States
         Securities and Exchange Commission and any stock exchange, self-
         regulatory or similar authority; and

     5)  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, (i) any of the undersigned's responsibilities to
comply with the requirements of the Exchange Act or any liability for the
undersigned's failure to comply with such requirements or (ii) any obligation or
liability that the undersigned incurs for profit disgorgement under Section
16(b) of the Securities Exchange Act of 1934, as amended.

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 does not revoke any other power of attorney that the
undersigned has previously granted.

IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed this 14th day of February, 2019.


                                        /s/ Angelos Stergiou
                                        --------------------
                                        Angelos Stergiou