POWER OF ATTORNEY

        The undersigned, as a Section 16 reporting person of ARMO BioSciences,
Inc., a Delaware corporation (the "Company") hereby constitutes and appoints
each of the persons listed on Exhibit A attached hereto, the undersigned's true
and lawful attorney-in-fact to:

        (1)   complete and execute Forms 3, 4 and 5 and other forms and all
amendments thereto as such attorney-in-fact shall in his or her discretion
determines to be required or advisable pursuant to Section 16 of the Securities
Exchange Act of 1934 (as amended) and the rules and regulations promulgated
thereunder, or any successor laws and regulations, as a consequence of the
undersigned's ownership, acquisition or disposition of securities of the
Company; and

        (2)   do all acts necessary in order to file such forms with the
Securities and Exchange Commission, any securities exchange or national
association, the Company and such other person or agency as the attorney-in-fact
shall deem appropriate.

        The undersigned hereby ratifies and confirms all that said attorneys-in-
fact and agents shall do or cause to be done by virtue hereof.  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 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.

        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 22 day of January, 2018.


                                        Alphabet Inc.

                                        By: /s/ Kent Walker
                                            ------------------------------------
                                        Name: Kent Walker
                                        Title: Assistant Secretary



                                       EXHIBIT A

                                     Marcia Hatch
                                      Heidi Mayon
                                     Colin Conklin
                                     Sogoal Salari
                                    Elaine Sylvester
                                     Scott Paraker
                                      Herb Cross