Exhibit 5.1

Goodwin Procter LLP 100 Northern Avenue Boston, MA 02210 goodwinlaw.com +1 617-570-1000

October 3, 2024

Allurion Technologies, Inc.

11 Huron Drive

Natick MA 01760

Re: Securities Registered under Registration Statement on Form S-1

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-1 (as amended or supplemented, the “Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of the sale by the selling securityholder listed in the Registration Statement under “Selling Securityholder” (the “Selling Securityholder”) of up to 65,211,325 shares of Common Stock, $0.0001 par value per share (“Common Stock”), of Allurion Technologies, Inc., a Delaware corporation (the “Company”), which are issuable to the Selling Securityholder, including: (i) 60,691,007 shares of Common Stock (the “Note Shares”) issuable upon the conversion of convertible senior secured notes (the “Convertible Notes”), (ii) 2,260,159 shares of Common Stock (the “Conversion Shares”) issuable upon conversion of Series A non-voting convertible preferred stock, par value $0.0001 per share (“Series A Preferred Stock”), and (iii) 2,260,159 shares of Common Stock (“Warrant Shares”) issuable upon the exercise of private placement warrants (“Private Placement Warrants”) to purchase Common Stock.

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinions set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinions set forth below, on certificates of officers of the Company. For purposes of the opinion set forth below, we have assumed that before the Shares are issued, the Company does not issue shares of Common Stock or reduce the total number of shares of Common Stock that the Company is authorized to issue under its certificate of incorporation such that the number of unissued shares of Common Stock authorized under the Company’s certificate of incorporation is less than the number of Shares.

The opinions set forth below are limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that:

1. The Note Shares have been duly authorized and, when issued and delivered upon the conversion of the Convertible Notes in accordance with the terms of the Convertible Notes, will be validly issued, fully paid and non-assessable.

2. The Conversion Shares have been duly authorized and, when issued and delivered upon the conversion of the Series A Preferred Stock in accordance with the terms of the certificate of designation to the Company’s certificate of incorporation, will be validly issued, fully paid and non-assessable.

3. The Warrant Shares have been duly authorized and, when issued upon exercise of the Private Warrants in accordance with the terms of the Private Warrants, will be validly issued, fully paid and nonassessable.

Our opinions set forth above are subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and other similar laws of general application affecting the rights and remedies of creditors and to general principles of equity.

This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

Allurion Technologies, Inc.

October 3, 2024

Page 2

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement and to the references to our firm under the caption “Legal Matters” in the Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,

/s/ Goodwin Procter LLP

GOODWIN PROCTER LLP