UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13G

(Rule 13d-102)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(b), (c) and (d) and Amendments Thereto Filed

Pursuant to § 240.13d-2

Under the Securities Exchange Act of 1934

(Amendment No.     )

Rapid Micro Biosystems, Inc.

(Name of Issuer)

Class A Common Stock, $0.01

par value per share

(Title of Class of Securities)

75340L104

(CUSIP Number)

December 31, 2021

(Date of Event Which Requires Filing of This Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934, as amended (“Act”), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

13G

13G

13G

Item 1(a). Name of Issuer

The name of the issuer to which this filing on Schedule 13G relates is Rapid Micro Biosystems, Inc. (the “Issuer”).

Item 1(b). Address of Issuer’s Principal Executive Offices

The principal executive offices of the Issuer are located at 1001 Pawtucket Boulevard West, Suite 280, Lowell, Massachusetts 01854.

Item 2(a). Name of Person Filing

This Schedule 13G is being filed jointly by Bain Capital Life Sciences Fund, L.P., a Cayman Islands exempted limited partnership (“BCLS”), and BCIP Life Sciences Associates, LP, a Delaware limited partnership (“BCIPLS” and, together with BCLS, the “Reporting Persons”).

Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company (“BCLSI”), is the general partner of Bain Capital Life Sciences Partners, LP, a Cayman Islands exempted limited partnership (“BCLSP”), which is the general partner of BCLS.

Boylston Coinvestors, LLC, a Delaware limited liability company (“Boylston”), is the general partner of BCIPLS. BCLSI 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 Reporting Persons.

The Reporting Persons have entered into a Joint Filing Agreement, dated February 14, 2022, pursuant to which the Reporting Persons have agreed to file this Schedule 13G jointly in accordance with the provisions of Rule 13d-1(k)(1) promulgated under the Act.

Item 2(b). Address of Principal Business Office or, if None, Residence

The principal business address for each of the Reporting Persons, BCLSI, BCLSP and Boylston is 200 Clarendon Street, Boston, Massachusetts 02116.

Item 2(c). Citizenship

BCLS and BCLSP are organized under the laws of the Cayman Islands. BCIPLS, BCLSI and Boylston are organized under the laws of the State of Delaware.

Item 2(d). Title of Class of Securities

The class of securities of the Issuer to which this Schedule 13G relates is Class A Common Stock, $0.01 par value per share (“Class A Common Stock”).

Item 2(e). CUSIP Number

The CUSIP number of the Class A Common Stock is 75340L104.

13G

Item 3. If this statement is filed pursuant to §§ 240.13d-1(b), or 240.13d-2(b) or (c), check whether the person filing is a

If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution.

Item 4. Ownership

(a) Amount beneficially owned:

As of the close of business on December 31, 2021, BCLS held 7,651,369 shares of Class A Common Stock, representing approximately 22.2% of the outstanding shares of Class A Common Stock, and BCIPLS held 783,191 shares of Class A Common Stock, representing approximately 2.3% of the outstanding shares of Class A Common Stock.

As a result of the foregoing and the relationships described in Item 2(a) of this Schedule 13G, the Reporting Persons may be deemed to collectively beneficially own an aggregate of 8,434,560 shares of Class A Common Stock, representing approximately 24.5% of the outstanding shares of Class A Common Stock.

The percentage of the outstanding shares of Class A Common Stock held by the Reporting Persons is based on 34,449,144 shares of Class A Common Stock issued and outstanding, as reported by the Issuer in its Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 15, 2021.

(b) Percent of class:

See Item 4(a) hereof.

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

0

(ii) Shared power to vote or direct the vote:

13G

(iii) Sole power to dispose or to direct the disposition of:

0

(iv) Shared power to dispose or to direct the disposition of:

Item 5. Ownership of Five Percent or Less of a Class

Not applicable.

Item 6. Ownership of More than Five Percent on Behalf of Another Person

Not applicable.

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not applicable.

Item 8. Identification and Classification of Members of the Group

Not applicable.

Item 9. Notice of Dissolution of Group

Not applicable.

Item 10. Certifications

Not applicable.

13G

SIGNATURES

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information in this statement is true, complete and correct.

Exhibit A

AGREEMENT REGARDING THE JOINT FILING OF

SCHEDULE 13G

The undersigned hereby agree as follows: