UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)

Westport Fuel Systems Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

960908309

(CUSIP Number)

Ryan J. York

Davis Wright Tremaine LLP

1201 Third Avenue, Suite 2200

Seattle, WA 98101

(206) 622-3150

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

June 1, 2016

(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 (“Act”) or otherwise subject to the liabilities of that section of Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 960908309

CUSIP No. 960908309

CUSIP No. 960908309

CUSIP No. 960908309

CUSIP No. 960908309

CUSIP No. 960908309

Schedule 13D

This Amendment No. 3 (“Amendment”) amends and supplements the Schedule 13D filed with the U.S. Securities and Exchange Commission (the “SEC”) on October 31, 2014, as previously amended by the Schedule 13D/A filed with the SEC on September 2, 2015, and as previously amended by the Schedule 13D/A filed with the SEC on March 21, 2016 (the “Schedule 13D”).

This Amendment is being filed solely to update the Filers’ beneficial ownership of shares of Common Stock following completion of the merger transactions effected June 1, 2016, among the Issuer, Fuel Systems Solutions, Inc. (“Fuel Systems”) and Whitehorse Merger Sub Inc., a Delaware corporation and wholly-owned subsidiary of the Issuer (“Merger Sub”), pursuant to which the shares of Fuel Systems were exchanged for shares of Common Stock of the Issuer on a one for 2.4755 basis. This Amendment reflects the beneficial ownership of the Filers as of June 1, 2016.

Except as set forth in this Amendment, the Schedule 13D remains unchanged. Unless otherwise indicated, all capitalized terms used herein but not defined herein shall have the same meaning as set forth in the Schedule 13D.

ITEM 1. SECURITY AND ISSUER

This statement relates to shares of Common Stock (the “Common Stock”) of Westport Fuel Systems Inc. (the “Issuer”) The principal executive office of the Issuer is located at Suite 101, 1750 West 75^th Avenue, Vancouver, British Columbia, Canada V6P 6G2.

ITEM 3. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION

On June 1, 2016, the Issuer acquired Fuel Systems pursuant to the Agreement and Plan of Merger by and among the Issuer, Merger Sub and Fuel Systems, dated as of September 1, 2015, as amended by Amendment No. 1 thereto, dated as of March 6, 2016 (the “Merger Agreement”). In accordance with the Merger Agreement, Merger Sub merged with and into Fuel Systems (the “Merger”), with Fuel Systems surviving the Merger as a wholly owned subsidiary of Westport. As a result of the Merger, an aggregate of 2,671,684 shares of common stock of Fuel Systems, comprising all of the Filers’ shares of common stock of Fuel Systems, were exchanged for shares of the Issuer’s Common Stock on a one for 2.4755 basis. The Filers were not required to expend any funds in connection with the foregoing exchange, and the Filers received the same consideration per share as other shareholders of Fuel Systems received pursuant to the Merger Agreement.

The Filers hereby add the following disclosure to this Item 4:

Item 3 is incorporated by reference in this Item 4 as if fully set forth herein. As described in Item 3 above, this Amendment updates the beneficial ownership of the Filers following completion of the Merger pursuant to the terms of the Merger Agreement. In addition, the Voting Agreement and the Letter Agreement terminated upon completion of the Merger. The Nomination Agreement remains in effect.

The Filers do not have any present plan or proposal that would relate to or result in any of the matters set forth in subparagraphs (a) – (j) of Item 4 of Schedule 13D, except as set forth herein or such as would occur upon completion of any of the actions discussed herein.

ITEM 5. INTEREST IN SECURITIES OF THE ISSUER

(a) and (b) The beneficial ownership of the Common Stock by each Filer at the date hereof is reflected on that Filer’s cover page.

(c) Except as set forth in this Amendment, there were no transactions effected by the Filers in the Common Stock within the past sixty (60) days.

(d) and (e) Not applicable.

ITEM 6. CONTRACTS, AGREEMENTS, UNDERSTANDINGS OR RELATIONSHIPS WITH RESPECT TO SECURITIES OF THE ISSUER.

Other than as described in Item 4 of this statement, to the knowledge of the Filers, none of the Filers is party to any other contracts, arrangements, understandings or relationships with respect to any securities of the Issuer, including but not limited to the transfer or voting of any of the securities, finder’s fees, joint ventures, loan or option agreements, puts or calls, guarantees of profits, division of profits or loss, or the giving or withholding of proxies.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.