POWER OF ATTORNEY

          Know all by these presents that the undersigned hereby constitutes and
appoints each of Rohan A. Palekar, Ryan Martins and Ryan A. Murr, and any of
their substitutes, signing singly, as the undersigned's true and lawful
attorney-in-fact to:

1.      prepare, execute in the undersigned's name and on the undersigned's
        behalf, and submit to the U.S. Securities and Exchange Commission (the
        "SEC") a Form ID, including amendments thereto, and any other documents
        necessary or appropriate to obtain codes, passwords, and passphrases
        enabling the undersigned to make electronic filings with the SEC of
        reports required by the Securities Exchange Act of 1934 or any rule or
        regulation of the SEC;

2.      execute for and on behalf of the undersigned, in the undersigned's
        capacity as an officer of 89bio, Inc. (the "Company"), Forms 3, 4, and 5
        in accordance with Section 16(a) of the Securities Exchange Act of 1934
        and the rules thereunder;

3.      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, complete and execute any amendment or amendments
        thereto, and timely file such form with the SEC and any securities
        exchange or similar authority; and

4.      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 each 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 also ratifies hereby any
action previously taken by each attorney-in-fact that would have been authorized
by this power of attorney if it has been in effect at the time such action was
taken.  The undersigned acknowledges that each attorney-in-fact, in serving in
such capacity at the request of the undersigned, is not assuming, nor is the
Company assuming, any of the undersigned's responsibilities to comply with
Section 16 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 each
of the foregoing attorneys-in-fact.

IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 9th day of October, 2019.

                                        /s/ Ram Waisbourd
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                                        Ram Waisbourd