As filed with the Securities and Exchange Commission on March 13, 2012

Registration No. 333-            

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

ENERGY RECOVERY, INC.

(Exact name of registrant as specified in its charter)

1717 Doolittle Drive, San Leandro, California 94577

(Address, including zip code, of registrant’s principal executive offices)

Energy Recovery, Inc.

2008 Equity Incentive Plan

(Full Title of the Plan)

Thomas S. Rooney, Jr.

President and Chief Executive Officer

1717 Doolittle Drive

San Leandro, CA 94577

(Name and Address of Agent for Service)

(510) 483-7370

(Telephone Number, Including Area Code, of Agent for Service)

Copy to:

Stephen J. Schrader, Esq.

Baker & McKenzie LLP

Two Embarcadero Center, 11th Floor

San Francisco, California 94111

(415) 576-3000

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Securities Exchange Act of 1934 (Check One):

CALCULATION OF REGISTRATION FEE

PART I

INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

The documents containing the information specified in Part I of Form S-8 (plan information and registrant information and employee plan annual information) will be sent or given to employees as specified by Rule 428(b)(1) under the Securities Act of 1933. Such documents need not be filed with the Securities and Exchange Commission (the “Commission”) either as part of this Registration Statement or as prospectuses or prospectus supplements pursuant to Rule 424 under the Securities Act of 1933. These documents and the documents incorporated by reference in this Registration Statement pursuant to Item 3 of Form S-8 (Part II hereof), taken together, constitute a prospectus that meets the requirements of Section 10(a) of the Securities Act of 1933.

PART II

INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.   Incorporation of Documents by Reference.

The following documents filed by Energy Recovery, Inc. (the “Registrant”) with the Commission are incorporated by reference herein:

All documents filed by the Registrant pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Securities Exchange Act of 1934 after the date of this Registration Statement, and prior to the filing of a post-effective amendment which indicates that all securities offered hereby have been sold or which deregisters all securities then remaining unsold, shall be deemed to be incorporated by reference in this Registration Statement and to be a part hereof from the date of filing of such documents.

Item 4.   Description of Securities.

Not applicable.

Item 5.   Interests of Named Experts and Counsel.

Not applicable.

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Item 6.   Indemnification of Directors and Officers.

The Registrant’s amended and restated certificate of incorporation provides that the Registrant may indemnify to the fullest extent permitted by Delaware law any person made or threatened to be made a party to an action or proceeding, whether criminal, civil, administrative or investigative, by reason of the fact that he or she was a director or officer of the Registrant or serves or served at any other enterprise as a director or officer at the request of the Registrant. Further, the Registrant’s amended and restated certificate of incorporation contains provisions that limit the personal liability of the Registrant’s directors for monetary damages to the fullest extent permitted by Delaware law.

In addition, as permitted by Section 145 of the Delaware General Corporation Law, the amended and restated bylaws of the Registrant provide that:

The Registrant also maintains directors and officers insurance to insure such persons against certain liabilities.

Item 7.   Exemption From Registration Claimed.

Not applicable.

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Item 8.   Exhibits.

Item 9.   Undertakings.

(A) The undersigned Registrant hereby undertakes:

(1) To file, during any period in which offers or sales are being made, a post-effective amendment to this Registration Statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities Act of 1933;

(ii) To reflect in the prospectus any facts or events arising after the effective date of the Registration Statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the Registration Statement;

(iii) To include any material information with respect to the plan of distribution not previously disclosed in the Registration Statement or any material change to such information in the Registration Statement;

Provided, however, that paragraphs (1)(i) and (1)(ii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the Registrant pursuant to section 13 or section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the Registration Statement.

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(2) That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(B) The undersigned Registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the Registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the Registration Statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(C) Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions or otherwise, the Registrant has been advised that in the opinion of the Commission such indemnification is against public policy as expressed in the Securities Act of 1933 and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the Registrant of expenses incurred or paid by a director, officer or controlling person of the Registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the Registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question of whether such indemnification by it is against public policy as expressed in the Securities Act of 1933 and will be governed by the final adjudication of such issue.

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the Registrant, Energy Recovery, Inc., certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of San Leandro, State of California, on this 13th day of March, 2012.

POWER OF ATTORNEY

Each person whose signature appears below constitutes and appoints Thomas S. Rooney, Jr. and Alexander J. Buehler and each of them, as attorneys-in-fact, each with the power of substitution, for him in any and all capacities, to sign any amendment to this Registration Statement and to file the same, with exhibits thereto and other documents in connection therewith, with the Securities and Exchange Commission, granting to said attorneys-in-fact full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully to all intents and purposes as he might or could do in person, hereby ratifying and confirming the said attorney-in-fact or his substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

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INDEX TO EXHIBITS