Common Stock, par value $0.001 per share

89BIO, INC.

142 Sansome Street, Second Floor, San Francisco, CA, 94104

This Amendment No. 13 is being filed on behalf of RA Capital Management, L.P. ("RA Capital"), Peter Kolchinsky, Rajeev Shah, and RA Capital Healthcare Fund, L.P. (the "Fund"), who are collectively referred to herein as the "Reporting Persons." The agreement among the Reporting Persons to file this 13D/A jointly in accordance with Rule 13d-1(k) of the Securities Exchange Act of 1934, as amended (the "Act"), is attached hereto as Exhibit 99.1. The Reporting Persons' ownership of the Issuer's securities includes (i) 19,554,319 shares of Common Stock held directly by the Fund; (ii) 335,364 shares of Common Stock held directly by RA Capital Nexus Fund, L.P. (the "Nexus Fund"); (iii) a total of 124,050 vested stock options (right to buy) held by Derek DiRocco for the benefit of RA Capital; and (iv) a total of 4,331,081 pre-funded warrants ("PFWs"), through which the Fund has the right to acquire 4,331,081 shares of Common Stock subject to the PFW Beneficial Ownership Blocker as defined below. The PFWs contain a provision (the "PFW Beneficial Ownership Blocker") which precludes exercise of the PFWs to the extent that, following the exercise, the Reporting Persons, together with their affiliates and other attribution parties, would beneficially own more than 9.99% of the Issuer's outstanding Common Stock. By virtue of the PFW Beneficial Ownership Blocker, the PFWs are currently not exercisable. RA Capital Healthcare Fund GP, LLC is the general partner of the Fund and RA Capital Nexus Fund GP, LLC is the general partner of the Nexus Fund. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund and the Nexus Fund and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund or the Nexus Fund. The Fund and the Nexus Fund have delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's and the Nexus Fund's portfolio, including the shares of the Issuer's Common Stock reported herein. Because the Fund and the Nexus Fund have divested themselves of voting and investment power over the reported securities they hold and may not revoke that delegation on less than 61 days' notice, the Fund and the Nexus Fund disclaim beneficial ownership of the securities they hold for purposes of Section 13(d) of the Act and therefore disclaim any obligation to report ownership of the reported securities under Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this 13D/A other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this 13D/A shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.

The address of the principal business office of each of the Reporting Persons is 200 Berkeley Street, 18th Floor, Boston, MA 02116.

The Fund is a private investment vehicle. RA Capital provides investment management services to the Fund and the Nexus Fund. The principal occupation of each of Dr. Kolchinsky and Mr. Shah is investment management.

During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree of final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

See Item 6 of the cover pages.

The information set forth in rows 11 and 13 of the cover pages to this Schedule 13D/A is incorporated by reference. The percentage set forth in row 13 is based on the sum of (i) 145,984,182 shares of Common Stock outstanding as of February 24, 2025, as reported in the Issuer's Annual Report on Form 10-K filed with the SEC on February 27, 2025; and (ii) 124,050 shares of Common Stock issuable upon exercise of stock options, as applicable.

The information set forth in rows 7 through 10 of the cover pages to this Schedule 13D/A and Item 2 above is incorporated by reference.

None of the Reporting Persons have effected any transactions with respect to the securities of the Issuer since the most recent amendment to this Schedule 13D/A. The percentage ownership reported in this amendment reflect an increase in the number of outstanding shares of the Issuer.

No person (other than the Reporting Persons) is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the shares of Common Stock subject to this 13D/A.

Not applicable.