Exhibit 5.1

February 28, 2014

Marin Software Incorporated

123 Mission Street, 25 ^th Floor

San Francisco, California 94105

Gentlemen/Ladies:

At your request, we have examined the Registration Statement on Form S-8 (the “ Registration Statement ”) to be filed by Marin Software Incorporated, a Delaware corporation (the “ Company ”), with the Securities and Exchange Commission (the “ Commission ”) on or about February 28, 2014 in connection with the registration under the Securities Act of 1933, as amended, of an aggregate of 1,977,174 shares of the Company’s common stock, $0.001 par value per share (the “ Stock ”), that are subject to issuance by the Company upon the exercise of (a) awards granted or to be granted under the Company’s 2013 Equity Incentive Plan (the “ 2013 Plan ”), and (b) purchase rights to acquire shares of common stock granted or to be granted under the Company’s 2013 Employee Stock Purchase Plan (the “ Purchase Plan ”). The 2013 Plan and the Purchase Plan are collectively referred to herein as the “ Plans .” In rendering this opinion, we have examined such matters of fact as we have deemed necessary in order to render the opinion set forth herein, which included examination of the following:

In our examination of documents for purposes of this opinion, we have assumed, and express no opinion as to, the genuineness of all signatures on original documents, the authenticity and completeness of all documents submitted to us as originals, the conformity to originals and completeness of all documents submitted to us as copies, the legal capacity of all persons or entities executing the same, the lack of any undisclosed termination, modification, waiver or amendment to any document reviewed by us and the due authorization, execution and delivery of all documents where due authorization, execution and delivery are prerequisites to the effectiveness thereof. We have also assumed that the certificates representing the Stock have been, or will be when issued, properly signed by authorized officers of the Company or their agents.

As to matters of fact relevant to this opinion, we have relied solely upon our examination of the documents referred to above and have assumed the current accuracy and completeness of the information obtained from the documents referred to above and the representations and warranties made by representatives of the Company to us, including but not limited to those set forth in the Opinion Certificate. We have made no independent investigation or other attempt to verify the accuracy of any of such information or to determine the existence or non-existence of any other factual matters.

We render this opinion only with respect to, and express no opinion herein concerning the application or effect of the laws of any jurisdiction other than, the existing laws of the United States of America, of the State of California and of the Delaware General Corporation Law and reported judicial decisions relating thereto.

With respect to our opinion expressed in paragraph (1) below as to the valid existence and good standing of the Company under the laws of the State of Delaware, we have relied solely upon the Certificate of Good Standing and representations made to us by the Company in the Opinion Certificate.

In accordance with Section 95 of the American Law Institute’s Restatement (Third) of the Law Governing Lawyers (2000), this opinion letter is to be interpreted in accordance with customary practices of lawyers rendering opinions in connection with the filing of a registration statement of the type described herein.

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

(1) The Company is a corporation validly existing, in good standing, under the laws of the State of Delaware; and

(2) The 1,977,174 shares of Stock that may be issued and sold by the Company upon the exercise of (a) awards granted or to be granted under the 2013 Plan, and (b) purchase rights granted or to be granted under the Purchase Plan, when issued, sold and delivered in accordance with the applicable plan and purchase agreements, if any, to be entered into thereunder and in the manner and for the consideration stated in the Registration Statement and relevant Prospectus, will be validly issued, fully paid and nonassessable.

We consent to the use of this opinion as an exhibit to the Registration Statement and further consent to all references to us, if any, in the Registration Statement, the Prospectus constituting a part thereof and any amendments thereto. In rendering the opinions set forth above, we are opining only as to the specific legal issues expressly set forth therein, and no opinion shall be inferred as to any other matter or matters.

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This opinion is intended solely for use in connection with issuance and sale of shares subject to the Registration Statement and is not to be relied upon for any other purpose. This opinion is rendered as of the date first written above and based solely on our understanding of facts in existence as of such date after the aforementioned examination. We assume no obligation to advise you of any fact, circumstance, event or change in the law or the facts that may hereafter be brought to our attention whether or not such occurrence would affect or modify any of the opinions expressed herein.