Exhibit 5.1

DLA Piper LLP (us)

2525 East Camelback Road, Suite 1000

Phoenix, Arizona 85016-4232

www.dlapiper.com

T    480.606.5100

F    480.606.5101

May 15, 2012

Board of Directors

Clean Diesel Technologies, Inc.

4567 Telephone Road, Suite 100

Ventura, CA 93003

Ladies and Gentlemen:

We serve as counsel to Clean Diesel Technologies, Inc., a Delaware corporation (the “Company”), and have been requested to render this opinion in connection with the Registration Statement on Form S-3 of the Company (as amended, the “Registration Statement”), filed with the Securities and Exchange Commission (the “SEC”) on May 15, 2012, including the preliminary prospectus included therein at the time the Registration Statement is declared effective (the “Prospectus”), under the Securities Act of 1933, as amended (the “Securities Act”), for registration by the Company of $50,000,000 aggregate initial offering price of Securities (as defined below).

As used herein, the term “Securities” includes (i) shares of common stock, par value $0.01 per share (the “Common Stock”), (ii) shares of preferred stock, par value $0.01 per share (the “Preferred Stock”), (iii) warrants to purchase Preferred Stock (the “Preferred Stock Warrants”) or Common Stock (the “Common Stock Warrants”), as designated by the Company at the time of the offering (collectively, the “Warrants”) and (iv) units comprised of one or more shares of Common Stock, shares of Preferred Stock and/or Warrants, in any combination (the “Units”). The Registration Statement provides that the Securities may be offered separately or together, in separate series, in amounts, at prices and on terms to be set forth in one or more supplements to the Prospectus (each, a “Prospectus Supplement”) or any related free writing prospectus (each, a “Free Writing Prospectus”).

In rendering the opinion expressed herein, we have reviewed originals or copies, certified or otherwise identified to our satisfaction, of the following documents (the “Documents”):

(1) The Registration Statement;

(2) the Restated Certificate of Incorporation, as amended (the “Charter”) and By-laws, as amended (the “By-Laws”), of the Company, certified as true, accurate and complete by an officer of the Company (collectively, the “Organizational Documents”);

(3) a Unanimous Written Consent of the Board of Directors, dated May 15, 2012, certified as true, accurate and complete, and in full force and effect, by an officer of the Company; and

(4) such other certificates, documents and matters as we have deemed necessary and appropriate to render the opinions set forth in this opinion, subject to the limitations, assumptions, and qualifications noted below.

In examining the Documents, we have assumed, without independent investigation, the genuineness of all signatures, the legal capacity of all individuals who have executed any of the aforesaid documents, the authenticity of all documents submitted to us as originals, and the conformity with originals of all documents submitted to us as copies (and the authenticity of the originals of such copies), the absence of other agreements or understandings among the parties that would modify the terms of the proposed transactions or the respective rights or obligations of the parties thereunder and the accuracy and

Board of Directors

Clean Diesel Technologies, Inc.

May 15, 2012

Page Two

completeness of all public records reviewed by us. In making our examination of documents executed by parties other than the Company, we have assumed that such parties had the power and authority (corporate, trust, partnership or other) to enter into and perform all obligations thereunder, and we have also assumed the due authorization by all requisite action (corporate, trust, partnership or other) and the valid execution and delivery by such parties of such documents and the validity, binding effect and enforceability thereof with respect to such parties. As to any facts material to this opinion, we have relied solely upon the Certificate.

We further assume that:

(1) The issuance, sale, amount and terms of the Securities to be offered from time to time by the Company will be authorized and determined by proper action of the Board of Directors (or where permitted, a committee of the Board of Directors) of the Company (each, a “Board Action”) in accordance with the Company’s Organizational Documents and applicable law, in each case so as not to result in a default under or breach of any instrument, document or agreement binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental or regulatory body having jurisdiction over the Company.

(2) The Company will not issue any Securities in excess of the number or amount authorized by a Board Action.

(3) Prior to the issuance of any shares of Preferred Stock (including Preferred Stock that is the subject of Preferred Stock Warrants), or Common Stock (including Common Stock that is the subject of Common Stock Warrants or convertible Preferred Stock), there will exist, under the Company’s Charter, the requisite number of authorized but unissued shares of Preferred Stock (and securities of any class into which any of the Preferred Stock may be convertible), or Common Stock, as the case may be, and that all actions necessary to the creation of any such Preferred Stock (and securities of any class into which any Preferred Stock may be convertible), whether by certificates of designation or by classification or reclassification of existing capital stock and the filing of amendments to the Company’s Charter, will have been taken.

(4) For shares of Preferred Stock or Common Stock represented by certificates (“Certificates”), appropriate Certificates representing shares of Preferred Stock or Common Stock will be executed and delivered upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter and By-Laws and applicable law. For shares of Preferred Stock or Common Stock not represented by certificates, the applicable Board Action shall have been taken and, upon request of a stockholder of the Company, appropriate written statements (“Written Statements”) will be prepared and delivered to such stockholder upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter and By-Laws and applicable law.

(5) Any Warrants will be issued under a valid and legally binding warrant agreement (a “Warrant Agreement”) that conforms to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus and will comply with the Company’s Charter and By-Laws and applicable law.

Board of Directors

Clean Desel Technologies, Inc.

May 15, 2012

Page Three

(6) Any Units will be issued under a valid and legally binding unit agreement (a “Unit Agreement”) that conforms to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus and will comply with the Company’s Charter and By-Laws and applicable law.

(7) The underwriting or other agreements for offerings of the Securities (each, an “Underwriting Agreement,” and collectively, the “Underwriting Agreements”) will be valid and legally binding contracts that conform to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus.

(8) The Registration Statement will have been declared effective and will remain effective under the Securities Act.

Based upon the foregoing, and subject to the assumptions, limitations and qualifications stated herein, it is our opinion that:

(a) Upon an issuance of Certificates or Written Statements, if any, as the case may be, representing shares of Common Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Charter, By-Laws and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, for shares of Common Stock pursuant to the exercise of one or more Common Stock Warrants or the conversion of one or more series of Preferred Stock convertible into Common Stock, such shares of Common Stock will be duly authorized, validly issued, fully paid and non-assessable.

(b) Upon an issuance of Certificates or Written Statements, if any, as the case may be, representing shares of Preferred Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Charter, By-Laws and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, for shares of Preferred Stock pursuant to the exercise of one or more Preferred Stock Warrants, such shares of Preferred Stock will be duly authorized, validly issued, fully paid and non-assessable.

(c) Upon execution, issuance, and delivery of the Warrants against payment therefor in accordance with the terms and provisions of the applicable Board Action, the Charter, the By-Laws and applicable law, the Warrant Agreement, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, the Warrants will be duly authorized and constitute valid and legally binding obligations of the Company.

Board of Directors

Clean Diesel Technologies, Inc.

May 15, 2012

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(d) Upon execution, issuance, and delivery of the Units against payment therefor in accordance with the terms and provisions of the applicable Board Action, the Charter, the By-Laws and applicable law, the Unit Agreement, the Registration Statement, the Prospectus, the applicable Prospectus Supplement, and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, and assuming that the underlying securities that are components of such Units have been duly and properly authorized for such issuance and constitute valid and binding obligations enforceable against the issuer thereof in accordance with their terms, the Units will be duly authorized and constitute valid and legally binding obligations of the Company.

In addition to the qualifications assumptions and limitations set forth above, the foregoing opinion is further qualified as follows:

(1) We do not express any opinion herein concerning any law other than the Delaware General Corporation Law and the substantive federal securities laws of the United States of America as in effect on the date hereof. We express no opinion as to the laws of any other state or jurisdiction of the United States or of any foreign jurisdiction. We have made no inquiry into the laws and regulations or as to laws relating to choice of law or conflicts of law principles. The opinion expressed herein is subject to the effect of judicial decisions which may permit the introduction of parol evidence to modify the terms or the interpretation of agreements.

(2) The opinion stated herein relating to the validity and binding nature of obligations of the Company is subject to (i) the effect of any applicable bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, fraudulent conveyance, moratorium or similar laws affecting creditors’ rights generally and (ii) the effect of general principles of equity (regardless of whether considered in a proceeding in equity or at law).

We hereby consent to the filing of this opinion with the SEC as Exhibit 5.1 to the Registration Statement and to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving our consent, we do not thereby 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 of the SEC thereunder.

Very truly yours,

DLA Piper LLP (US)