Common Stock, par value $0.0001 per share

Climb Bio, Inc.

20 William Street, Suite 145, Wellesley Hills, MA, 02481

This Schedule 13D/A is being filed on behalf of RA Capital, Dr. Kolchinsky, Mr. Shah, the Fund and the Nexus Fund III. The foregoing entities and individuals are collectively referred to herein as the "Reporting Persons." The agreement among the Reporting Persons to file this Schedule 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' beneficial ownership of the Issuer's securities includes (i) 3,837,101 shares of common stock held directly by the Fund, (ii) pre-funded warrants (the "Pre-Funded Warrants") exercisable for up to 22,546,000 shares of common stock held directly by the Fund; (iii) 1,226,497 shares of common stock held directly by RA Capital Nexus Fund, L.P. (the "Nexus Fund"); (iv) 483,679 shares of common stock held directly by RA Capital Nexus Fund II, L.P. (the "Nexus Fund II"); (v) 3,671,766 shares of common stock held directly by the Nexus Fund III; (vi) 841,087 shares of common stock held directly by a separately managed account (the "Account"); (vii) 90,000 vested stock options (right to buy) held by Dr. Levin for the benefit of RA Capital; and (viii) 3,904 stock options (right to buy) scheduled to vest within 60 days of this filing held by Dr. Celebi for the benefit of RA Capital. The Pre-Funded Warrants contain a provision (the "Beneficial Ownership Blocker") which precludes exercise of the Pre-Funded Warrants to the extent that, following exercise, the Fund, together with its affiliates and other attribution parties, would own more than 33.0% of the common stock outstanding. The Fund is currently prohibited from exercising the Pre-Funded Warrants to the extent that such exercise would result in the Reporting Persons' beneficial ownership of more than 23,249,186 shares of common stock. RA Capital Healthcare Fund GP, LLC is the general partner of the Fund, RA Capital Nexus Fund GP, LLC is the general partner of the Nexus Fund, RA Capital Nexus Fund II GP, LLC is the general partner of the Nexus Fund II, and RA Capital Nexus Fund III GP, LLC is the general partner of the Nexus Fund III. 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, the Nexus Fund, the Nexus Fund II, the Nexus Fund III and the Account 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, the Nexus Fund, the Nexus Fund II, the Nexus Fund III or the Account. The Fund, the Nexus Fund, the Nexus Fund II and the Nexus Fund III have delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund, the Nexus Fund, the Nexus Fund II and the Nexus Fund III, and each of these funds has 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, the Nexus Fund, the Nexus Fund II and the Nexus Fund III 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 Schedule 13D/A other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of the Schedule 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 and the Nexus Fund III are private investment vehicles. RA Capital provides investment management services to the Fund, the Nexus Fund, the Nexus Fund II, the Nexus Fund III and the Account. The principal occupation 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.

Rows 11 and 13 of each Reporting Person's cover page to this Schedule 13D/A set forth the aggregate number of shares of common stock and percentages of the shares of common stock beneficially owned by such Reporting Person and are incorporated by reference. The percentage set forth in each row 13 is based upon 57,263,023 shares of common stock outstanding as of May 14, 2026, as reported in the Issuer's Registration Statement on Form S-3 filed with the SEC on May 29, 2026, and giving effect to stock options and Pre-Funded Warrants, to the extent exercisable within 60 days hereof, as referenced herein.

Rows 7 through 10 of each Reporting Person's cover page to this Schedule 13D/A set forth the number of shares of common stock as to which such Reporting Person has the sole or shared power to vote or direct the vote and sole or shared power to dispose or to direct the disposition and are incorporated by reference.

On July 2, 2026, Sera made a pro rata in-kind distribution for no additional consideration of 2,479,872 shares of common stock to its members. Of the shares distributed by Sera, the Fund and the Nexus Fund III received 433,672 shares and 805,391 shares, respectively. Except as set forth above and in the table below, none of the Reporting Persons has effected any transactions with respect to the securities of the Issuer during the past sixty days. Reporting No. Price Per Transaction Person Date Shares Share Vest Stock Option (Right to Buy) RA Capital 07/29/2026 1,952 (1) (2) Vest Stock Option (Right to Buy) RA Capital 08/29/2026 1,952 (1) (2) (1) This option represents a right to purchase a total of 70,284 shares of the Issuer's common stock, which shall vest in thirty-six equal monthly installments from June 29, 2026, subject to Dr. Celebi's continuous service to the Issuer through each such vesting date. These options have an exercise price of $13.36. (2) Represents future vesting options within 60 days from the filing date of this Schedule 13D/A.

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 securities subject to this Schedule 13D/A.

Not applicable.