Exhibit 5.1

Pillsbury Winthrop Shaw Pittman LLP

2550 Hanover Street | Palo Alto, CA 94304-1115 | tel 650.233.4500 | fax 650.233.4545

August 8, 2014

MoSys, Inc.

3301 Olcott Street

Santa Clara, CA 95054

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

We are acting as counsel for MoSys, Inc. a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-3 relating to the registration under the Securities Act of 1933 (the “Act”) of the following securities of the Company having an aggregate initial public offering price of up to U.S. $50,000,000, or the equivalent thereof in foreign currencies: (a) shares of common stock, par value $.0.01 per share (“Common Stock”), (b) shares of preferred stock, par value $0.01 per share (“Preferred Stock”), in one or more series, (c) senior or subordinated debt securities (“Debt Securities”), and (d) warrants to purchase shares of Common Stock or Preferred Stock or Debt Securities (“Warrants”). The Common Stock, Preferred Stock, Debt Securities and Warrants are collectively referred to as the “Securities”. The Securities shall include any additional amounts of such securities the offer and sale of which are registered pursuant to a registration statement filed pursuant to Rule 462(b) under the Act in connection with one or more offerings contemplated by such Registration Statement. Such Registration Statement, as amended, and including any registration statement related thereto and filed pursuant to Rule 462(b) under the Act, is herein referred to as the “Registration Statement.” The Debt Securities will be issued under an indenture in substantially the form of Exhibit 4.3 to the Registration Statement (the “Indenture”) to be entered into between the Company and a trustee to be identified in the Indenture (the “Trustee”).

We have reviewed and are familiar with such corporate proceedings and other matters as we have deemed necessary for the opinions expressed in this letter. In such review, we have assumed that the Indenture will be duly authorized, executed and delivered by the applicable Trustee, where applicable, the Securities will be properly authenticated by the manual signature of an authorized representative of the Trustee, warrant agent, depositary or transfer agent, and the signatures on all documents examined by us are genuine, which assumptions we have not independently verified.

On the basis of the assumptions and subject to the qualifications and limitations set forth herein, we are of the opinion that

1. With respect to the Common Stock, when the board of directors of the Company or a duly authorized committee of such board (such board of directors or committee being referred to herein as the “Board”) has taken all necessary corporate action to approve the issuance and establish the terms of the offering of shares of the Common Stock and related matters, and when such shares have been issued and sold by the Company in the manner contemplated by the Registration Statement and in accordance with such Board action, such shares of Common Stock (including any Common Stock duly issued upon conversion, exchange or exercise of any other Security in accordance with the terms of such other Security or the instrument governing such other Security providing for such conversion, exchange or exercise as approved by the Board) will be duly authorized, legally issued, fully paid and nonassessable.

2. With respect to the Preferred Stock, when the Board has taken all necessary corporate action to approve the issuance and establish the terms of any particular series of Preferred Stock, the offering thereof and related matters, including the filing of a certificate of designations conforming to the General Corporation Law of the State of Delaware regarding such series of Preferred Stock with the Secretary of State of the State of Delaware, and when shares of such series of Preferred Stock have been issued and sold by the Company in the manner contemplated by the Registration Statement and in accordance with such Board action, such shares of such series of Preferred Stock (including any shares of such series of Preferred Stock duly issued upon conversion, exchange or exercise of any other Security in accordance with the terms of such other Security or the instrument governing such other Security providing for such conversion, exchange or exercise as approved by the Board) will be duly authorized, legally issued, fully paid and nonassessable.

3. With respect to any of the Debt Securities, when (a) the Indenture has been duly authorized, executed and delivered by the Company and duly qualified under the Trust Indenture Act of 1939, (b) the Board has taken all necessary corporate action to approve the issuance and establish the terms of such Debt Securities, the terms of the offering and related matters, (c) such Debt Securities have been duly executed and authenticated in accordance with the terms of the Indenture, and (d) such Debt Securities have been issued and sold in the manner contemplated by the Registration Statement and in accordance with the Indenture, such Debt Securities (including any Debt Securities duly issued upon conversion, exchange or exercise of any other Security in accordance with the terms of such other Security or the instrument governing such other Security providing for such conversion, exchange or exercise as approved by the Board) will constitute the valid and legally binding obligations of the Company, enforceable against the Company in accordance with their terms.

4. With respect to any of the Warrants, when (a) one or more agreements incorporating the terms and other provisions thereof has been duly executed and delivered by the Company and a warrant agent (a “Warrant Agreement”), (b) the Board has taken all necessary corporate action to approve the issuance and establish the terms of such Warrants, the

terms of the offering of such Warrants, and related matters, (c) the Warrant certificates have been duly executed and authenticated or countersigned in accordance with the terms of such Warrant Agreement, and (d) such Warrants have been issued and sold in the manner contemplated by the Registration Statement and in accordance with such Warrant Agreement, such Warrants will constitute the valid and legally binding obligations of the Company, enforceable against the Company in accordance with their terms.

Our opinions set forth above are subject to and limited by the effect of (a) applicable bankruptcy, insolvency, fraudulent conveyance, fraudulent transfer, receivership, conservatorship, arrangement, moratorium and other laws affecting and relating to the rights of creditors generally, (b) general equitable principles, and (c) requirements of reasonableness, good faith, fair dealing and materiality.

In connection with the opinions expressed above, we have assumed that (a) at or prior to the time of the delivery of any such Security, the Registration Statement, and any amendments thereto (including post-effective amendments) will have been declared effective under the Act and a Prospectus Supplement to the Prospectus forming a part of the Registration Statement will have been prepared and filed with the Securities and Exchange Commission (the “Commission”) describing the Securities offered thereby, (b) at or prior to the time of the issuance of any such Security, the authorization of such Security will not have been modified or rescinded by the Board, (c) in the case of the issuance of shares of the Common Stock or the Preferred Stock, the Company will have a sufficient number of authorized but unissued shares therefor under its certificate of incorporation at the time of such issuance, (d) there will not have occurred any change in law affecting the validity or enforceability of such Security, and (e) none of the terms of any Security to be established subsequent to the date hereof nor the issuance and delivery of such Security, nor the compliance by the Company with the terms of such Security, will violate any applicable federal or state law or will result in a violation of any provision of any instrument or agreement then binding upon the Company or any restriction imposed by any court or governmental body having jurisdiction over the Company.

The opinions set forth in this letter are limited to the General Corporation Law of the State of Delaware and the law of the State of New York, in each case as in effect on the date hereof.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Legal Matters” in the Registration Statement and in the Prospectus forming a part thereof and any supplement thereto. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Pillsbury Winthrop Shaw Pittman LLP