Sec Form 4 Filing - Bain Capital Life Sciences Investors, LLC @ RAPID MICRO BIOSYSTEMS, INC. - 2026-05-20

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

1. Name and Address of Reporting Person Bain Capital Life Sciences Investors, LLC 2. Issuer Name and Ticker or Trading Symbol RAPID MICRO BIOSYSTEMS, INC. [ RPID] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 200 CLARENDON STREET 3. Date of Earliest Transaction (MM/DD/YY) 05/20/2026 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Class A Common Stock 8,434,560 I See footnotes ( 4 ) ( 5 ) ( 6 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Pre-Funded Warrant (right to buy) $ 0.01 05/20/2026 P( 1 ) 1,463,000 05/20/2026( 2 ) ( 3 ) Class A Common Stock 1,463,000 ( 2 ) ( 1 ) 1,463,000 I See footnotes ( 1 ) ( 6 ) Series A Warrant (right to buy) $ 1.955 05/20/2026 P( 1 ) 1,463,000 11/20/2026( 2 ) 05/20/2027 Class A Common Stock 1,463,000 ( 2 ) ( 1 ) 1,463,000 I See footnotes ( 1 ) ( 6 ) Series B Warrant (right to buy) $ 2.34 05/20/2026 P( 1 ) 1,463,000 11/20/2026( 2 ) 05/20/2031 Class A Common Stock 1,463,000 ( 2 ) ( 1 ) 1,463,000 I See footnotes ( 1 ) ( 6 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Bain Capital Life Sciences Investors, LLC 200 CLARENDON STREET BOSTON, MA02116 X Bain Capital Life Sciences Partners, LP 200 CLARENDON STREET BOSTON, MA02116 X Bain Capital Life Sciences Fund, L.P. 200 CLARENDON STREET BOSTON, MA02116 X BCIP Life Sciences Associates, LP 200 CLARENDON STREET BOSTON, MA02116 X Signatures See signatures included in Exhibit 99.1 05/22/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On May 20, 2026, BCLS I Investco, LP ("BCLS I Investco") purchased, in an underwritten public offering, a Pre-Funded Warrant to purchase 1,463,000 shares of the Issuer's Class A Common Stock, as well as an accompanying Series A Warrant to purchase 1,463,000 shares of the Issuer's Class A Common Stock (or pre-funded warrants in lieu thereof) and an accompanying Series B Warrant to purchase 1,463,000 shares of the Issuer's Class A Common Stock (or pre-funded warrants in lieu thereof). The purchase price of each pre-funded warrant and accompanying Series A Warrant and Series B Warrant was $1.945. ( 2 )The Pre-Funded Warrant, Series A Warrant and Series B Warrant cannot be exercised for shares of the Issuer's Class A Common Stock if, as a result of such exercise, the holder, together with its affiliates and any other persons whose beneficial ownership would be aggregated for purposes of Section 13(d) of the Act, would beneficially own more than 9.99% of the total number of shares of the Issuer's Class A Common Stock then outstanding immediately following such exercise. ( 3 )The Pre-Funded Warrant does not expire. ( 4 )Represents 7,651,369 shares of the Issuer's Class A Common Stock held directly by Bain Capital Life Sciences Fund, L.P. ("BCLS Fund I") and 783,191 shares of the Issuer's Class A Common Stock held directly by BCIP Life Sciences Associates, LP ("BCIPLS," and together with BCLS Fund I and BCLS I Investco, the "Bain Capital Life Sciences Entities"). ( 5 )Bain Capital Life Sciences Partners, LP ("BCLSP") is the general partner of BCLS Fund I. As a result, BCLSP may be deemed to share voting and dispositive power with respect to the securities held by BCLS Fund I. BCLSP disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein. ( 6 )Bain Capital Life Sciences Investors, LLC ("BCLSI") is the general partner of each of BCLSP and BCLS I Investco and governs the investment strategy and decision-making process with respect to investments held by BCIPLS. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Bain Capital Life Sciences Entities. BCLSI disclaims beneficial ownership of such securities except to the extent of its pecuniary interest therein.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.