Exhibit 5.1

August 5, 2013

Advent Software, Inc.

600 Townsend Street

San Francisco, California 94103

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

At your request, we have examined the Registration Statement on Form S-3 (the “Registration Statement”), to be filed by Advent Software, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”) in connection with the registration pursuant to the Securities Act of 1933, as amended (the “Act”), of the Common Stock (as defined below). The Registration Statement relates to the proposed issuance and sale, from time to time pursuant to Rule 415 under the Act as set forth in the Registration Statement, the prospectus contained therein (the “Prospectus”) and the supplements to the prospectus referred to therein (the “Prospectus Supplements”), of up to 8,351,121 shares of the Company’s common stock, $0.01 par value per share (the “Common Stock”). The shares of Common Stock are to be sold pursuant to an underwriting agreement in substantially the form to be filed as an exhibit to the Registration Statement.

We have examined instruments, documents, certificates and records which we have deemed relevant and necessary for the basis of our opinion hereinafter expressed. In such examination, we have assumed (i) the authenticity of original documents and the genuineness of all signatures; (ii) the conformity to the originals of all documents submitted to us as copies; (iii) the truth, accuracy, and completeness of the information, representations and warranties contained in the records, documents, instruments and certificates we have reviewed; (iv) the Registration Statement, and any amendments thereto (including post-effective amendments), will have become effective under the Act; (v) a prospectus supplement will have been filed with the Commission describing the Common Stock offered thereby; (vi) all Common Stock will be issued and sold in compliance with applicable Federal and state securities laws and in the manner stated in the Registration Statement and the applicable prospectus supplement; and (vii) a definitive purchase, underwriting or similar agreement with respect to any Common Stock offered will have been duly authorized and validly executed and delivered by the selling stockholders, the Company and the other parties thereto.

Based on such examination, and subject to the qualifications set forth herein, we are of the opinion that the shares of Common Stock offered by the Registration Statement will be validly issued, fully paid and nonassessable.

We express no opinion herein as to the laws of any state or jurisdiction other than the general Corporation Law of the State of Delaware (including the statutory provisions and all applicable judicial decisions interpreting those laws) and the Federal laws of the United States of America.

We consent to the use of this opinion as an exhibit to the Registration Statement, and we consent to the reference of our name under the caption “Legal Matters” in the prospectus filed as part of the Registration Statement.

Very truly yours,

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation

/s/ Wilson Sonsini Goodrich & Rosati