POWER OF ATTORNEY

        Know all by these presents, that the undersigned hereby constitutes and
appoints Laura Holm, Seth Goettelman or John Simard with full power of
substitution, as 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 XBiotech Inc. or any
          successor thereto (the "Company"), Forms 3, 4, and 5 in accordance
          with Section 16(a) of the Securities Exchange Act of 1934 and the
          rules thereunder (the "Exchange Act"), Form 144 in accordance with
          Rule 144 under the Securities Act of 1933 ("Rule 144") and any other
          forms or reports, including, but not limited to, a Form ID, that the
          undersigned may be required to file in connection with the
          undersigned's ownership, acquisition or disposition of securities of
          the Company;

     (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, Form 144 or other form or report, complete and execute
          any amendment or amendments thereto and timely file such form or
          report with the 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 of the Exchange Act or
Rule 144.

        This Power of Attorney revokes any previous Power of Attorney filed with
the Company for the purposes set forth herein and 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 expressly revoked before that time.

        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 13th day of April, 2015.

                                        /s/ W. Thorpe Mckenzie
                                        ----------------------------------------
                                        W. Thorpe Mckenzie