UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13D

(Rule 13d-101)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(a) and Amendments Thereto Filed

Pursuant to § 240.13d-2(a)

Under the Securities Exchange Act of 1934

(Amendment No. 4)

Solid Biosciences Inc.

(Name of Issuer)

Common Stock, $0.001

par value per share

(Title of Class of Securities)

83422E 105

(CUSIP Number)

Bain Capital Life Sciences Investors, LLC

200 Clarendon Street

Boston, MA 02116

617-516-2000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

August 18, 2022

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

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).

This Amendment No. 4 to Schedule 13D relates to the Common Stock of Solid Biosciences Inc. and amends the initial statement on Schedule 13D filed by the Reporting Person on February 1, 2018, as amended by Amendment No. 1 filed on July 30, 2019, Amendment No. 2 filed on December 11, 2020 and Amendment No. 3 filed on March 25, 2021 (the “Initial Statement” and, as further amended by this Amendment No. 4, the “Schedule 13D”). Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Initial Statement.

Item 2. Identity and Background

Item 2 of the Initial Statement is hereby amended and restated as follows:

(a) This Schedule 13D is being filed by BCLS SB Investco, LP, a Delaware limited partnership (the “Reporting Person”).

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 exempted limited partnership (“BCLSP” and, together with the Reporting Person and BCLSI, the “Bain Capital Life Sciences Entities”), which is the general partner of the Reporting Person. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Person.

(b) The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, MA 02116.

(c) Each of the Bain Capital Life Sciences Entities is principally engaged in the business of investment in securities.

(d) During the last five years, none of the Bain Capital Life Sciences Entities has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) During the last five years, none of the Bain Capital Life Sciences Entities 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 or final order enjoining future violation of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) The Reporting Person and BCLSI are organized under the laws of the State of Delaware. BCLSP is organized under the laws of the Cayman Islands.

Item 4. Purpose of Transaction

Item 4 of the Initial Statement is hereby amended and supplemented to add the following:

As previously disclosed by the Issuer, Dr. Adam Koppel, a Managing Director of BCLSI, did not stand for re-election to the Issuer’s board of directors (the “Board”) at the Issuer’s 2022 annual meeting of shareholders. Nevertheless, the Reporting Person is and will continue to be engaged collaboratively and constructively with the Issuer’s management and Board on operational, financial and/or strategic initiatives, including, without limitation, exploring, evaluating, proposing and/or negotiating one or more acquisitions and/or significant financing transactions in which the Reporting Person or its affiliates acquires additional securities of the Issuer and seeks to have one or more representatives serve on the

Board. The Reporting Person reviews and intends to continue to review, on an ongoing basis, its investment in the Issuer. Depending upon the factors discussed below and subject to applicable law, the Reporting Person may from time to time directly or indirectly acquire additional securities of the Issuer, sell or otherwise dispose of some or all of its securities of the Issuer and/or cease any of the activities described above. Any action taken by the Reporting Person may be made at any time and from time to time without prior notice and will depend on a variety of factors, including, without limitation, current and anticipated future trading prices of the securities of the Issuer, the financial condition, results of operations and prospects of the Issuer, available financing sources, general economic, financial market and industry conditions, other investment and business opportunities available to the Reporting Person, tax considerations and other factors.

Item 5. Interest in Securities of the Issuer

Item 5 of the Initial Statement is hereby amended and restated as follows:

The information set forth in or incorporated by reference in Items 2 and 3 and on the cover pages of this Schedule 13D is incorporated by reference in its entirety into this Item 5.

SIGNATURES

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