SCHEDULE 13D/A 0001687880 XXXXXXXX LIVE 6 Common Stock, par value $0.0001 per share 01/07/2026 false 0001826457 61225M102 Monte Rosa Therapeutics, Inc.

321 Harrison Avenue Suite 900 Boston MA 02118

Max Eisenberg 415-801-8100 One Sansome Street, Suite 1650 San Francisco CA 94104 0001687880 N Versant Venture Capital VI, L.P. b WC N DE 2236708 0 2236708 0 2236708 N 3.4 PN All shares are held by Versant VI (as defined in Item 2(a) of the Original Schedule 13D). Versant Ventures VI GP-GP (as defined in Item 2(a) of the Original Schedule 13D) is the general partner of Versant Ventures VI GP (as defined in Item 2(a) of the Original Schedule 13D), which is the general partner of Versant VI. Each of Versant Ventures VI GP-GP and Versant Ventures VI GP may be deemed to share voting, investment and dispositive power with respect to the shares held by Versant VI. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock (as defined in Item 1 of the Original Schedule 13D) outstanding as of October 31, 2025, as set forth in the Issuer's quarterly report on Form 10-Q for the quarter ended September 30, 2025, filed with the United States Securities and Exchange Commission (the "Commission") on November 6, 2025 (the "Form 10-Q"). 0001777654 N Versant Ventures VI GP, L.P. b AF N DE 0 2236708 0 2236708 2236708 N 3.4 PN All shares are held by Versant VI. Versant Ventures VI GP-GP is the general partner of Versant Ventures VI GP, which is the general partner of Versant VI. Each of Versant Ventures VI GP-GP and Versant Ventures VI GP may be deemed to share voting, investment and dispositive power with respect to the shares held by Versant VI. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. 0001777652 N Versant Ventures VI GP-GP, LLC b AF N DE 0 2236708 0 2236708 2236708 N 3.4 OO All shares are held by Versant VI. Versant Ventures VI GP-GP is the general partner of Versant Ventures VI GP, which is the general partner of Versant VI. Each of Versant Ventures VI GP-GP and Versant Ventures VI GP may be deemed to share voting, investment and dispositive power with respect to the shares held by Versant VI. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. Y Versant Vantage I, L.P. b WC N DE 1013361 0 1013361 0 1013361 N 1.6 PN All shares are held by Versant Vantage I (as defined in item 2(a) of the Original Schedule 13D). Versant Vantage I GP (as defined in item 2(a) of the Original Schedule 13D) is the general partner of Versant Vantage I, and Versant Vantage I GP-GP (as defined in item 2(a) of the Original Schedule 13D) is the general partner of Versant Vantage I GP. Each of Versant Vantage I GP and Versant Vantage I GP-GP share voting, investment and dispositive power over the shares held by Versant Vantage I. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. Y Versant Vantage I GP, L.P. b AF N DE 0 1013361 0 1013361 1013361 N 1.6 PN All shares are held by Versant Vantage I. Versant Vantage I GP is the general partner of Versant Vantage I, and Versant Vantage I GP-GP is the general partner of Versant Vantage I GP. Each of Versant Vantage I GP and Versant Vantage I GP-GP share voting, investment and dispositive power over the shares held by Versant Vantage I. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. Y Versant Vantage I GP-GP, LLC b AF N DE 0 1013361 0 1013361 1013361 N 1.6 OO All shares are held by Versant Vantage I. Versant Vantage I GP is the general partner of Versant Vantage I, and Versant Vantage I GP-GP is the general partner of Versant Vantage I GP. Each of Versant Vantage I GP and Versant Vantage I GP-GP share voting, investment and dispositive power over the shares held by Versant Vantage I. The percentage in Row 13 is based on 65,117,761 shares of the Issuer's Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. Common Stock, par value $0.0001 per share Monte Rosa Therapeutics, Inc. 321 Harrison Avenue Suite 900 Boston MA 02118 This Amendment No. 6 (this "Amendment") amends and supplements the Schedule 13D originally filed by the Reporting Persons with the Commission on July 8, 2021, as amended by Amendment No. 1 filed with the Commission on November 9, 2023, Amendment No. 2 filed with the Commission on August 12, 2024, Amendment No. 3 filed with the Commission on September 24, 2024, Amendment No. 4 filed with the Commission on October 30, 2024, and Amendment No. 5 filed with the Commission on November 12, 2025 (collectively, the "Original Schedule 13D"). Only those items that are hereby reported are amended; all other items reported in the Original Schedule 13D remain unchanged. Information given in response to each item shall be deemed incorporated by reference in all other items, as applicable. Capitalized terms not defined in this Amendment have the meanings ascribed to them in the Original Schedule 13D. The information reported in this Amendment is based on the Reporting Persons' holdings as of January 8, 2026. The aggregate percentage ownership of the Reporting Persons is 4.99%, calculated by taking 3,250,069 aggregate shares of Common Stock held by the Reporting Persons, which consists of (i) 2,236,708 shares of Common Stock held by Versant VI plus (ii) 1,013,361 shares of Common Stock held by Versant Vantage I and dividing it by 65,117,761 shares of Common Stock outstanding as of October 31, 2025, as reported by the Issuer in the Form 10-Q. See Items 7-11 and 13 of the cover pages of this Amendment for each Reporting Person and the corresponding comments. See Items 7-11 and 13 of the cover pages of this Amendment for each Reporting Person and the corresponding comments. On November 19, 2025, Versant VI sold 15,200 shares of Common Stock at a weighted average price per share of $14.2503 for aggregate proceeds of approximately $216,605.02. On December 4, 2025, Versant VI effected a pro rata distribution without additional consideration of 500,030 shares of Common Stock to its limited partners. On December 16, 2025, Versant VI sold 10,205 shares of Common Stock at a price per share of $17.64 for aggregate proceeds of approximately $180,016.20. On January 7, 2026, Versant VI sold 572,526 shares of Common Stock at a weighted average price per share of $23.3304 for aggregate proceeds of approximately $13,357,260.59. On January 8, 2026, Versant Vantage I effected a pro rata distribution without additional consideration of 560,092 shares of Common Stock to its limited partners. January 8, 2026 Versant Venture Capital VI, L.P. /s/Max Eisenberg 01/09/2026 Versant Ventures VI GP, L.P. /s/Max Eisenberg 01/09/2026 Versant Ventures VI GP-GP, LLC /s/Max Eisenberg 01/09/2026 Versant Vantage I, L.P. /s/Max Eisenberg 01/09/2026 Versant Vantage I GP, L.P. /s/Max Eisenberg 01/09/2026 Versant Vantage I GP-GP, LLC /s/Max Eisenberg 01/09/2026