Exhibit 3.1

Amendment to

Amended and Restated By-laws

of

Acacia Communications, Inc.

The Amended and Restated By-laws of Acacia Communications, Inc. be and hereby are amended by adding thereto the following provision as a new Section 5.9 thereof:

“5.9. Forum Selection By-law. Unless the corporation consents in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware (or, if the Court of Chancery does not have jurisdiction, the United States District Court for the District of Delaware) shall, to the fullest extent permitted by law, be the sole and exclusive forum for any action by a stockholder of the corporation (in their capacity as such) that is: (a) any derivative action or proceeding brought on behalf of the corporation, (b) any action asserting a claim of breach of a fiduciary duty owed by any current or former director, officer, other employee, agent or stockholder of the corporation to the corporation or the corporation’s stockholders, including, without limitation, a claim alleging the aiding and abetting of such a breach of fiduciary duty, (c) any action asserting a claim arising pursuant to any provision of the General Corporation Law of the State of Delaware, the Certificate of Incorporation or these By-laws (as each may be amended from time to time) or as to which the General Corporation Law of the State of Delaware confers jurisdiction on the Court of Chancery of the State of Delaware, (d) any action against the corporation or any director or officer or other employee of the corporation to interpret, apply, enforce or determine the validity of the Certificate of Incorporation or these By-laws (as each may be amended from time to time) or (e) any action asserting a claim governed by the internal affairs doctrine or other “internal corporate claim” as that term is defined in Section 115 of the General Corporation Law of the State of Delaware. Any person or entity purchasing or otherwise acquiring or holding any interest in shares of capital stock of the corporation shall be deemed to have notice of and to have consented to the provisions of this Section 5.9.”

Adopted by the Board of Directors of Acacia Communications, Inc. on July 8, 2019.