Exhibit 5.1

Opinion of Katten Muchin Rosenman LLP, the legal counsel to the Company

May 11, 2020

Information Services Group, Inc.

2187 Atlantic Street

Stamford, CT 06902

Ladies and Gentlemen:

We have acted as counsel to Information Services Group, Inc., a Delaware corporation (the “Company”), in connection with the filing of a Registration Statement on Form S-8 (the “Registration Statement”) for the registration under the Securities Act of 1933, as amended (the “Act”), of the issuance and sale by the Company of (i) up to an additional 5,500,000 shares (the “Incentive Plan Shares”) of the Company’s common stock, $0.001 par value per share (the “Common Stock”), pursuant to the Information Services Group, Inc. Amended and Restated 2007 Equity and Incentive Award Plan (the “Incentive Plan”), and (ii) up to an additional 1,200,000 shares (the “ESPP Shares” and together with the Incentive Plan Shares, the “Shares”) of the Company’s Common Stock pursuant to the Information Services Group, Inc. Amended and Restated 2007 Employee Stock Purchase Plan (the “ESPP” and together with the Incentive Plan, the “Plans”). This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Act.

In connection with this opinion, we have relied as to matters of fact, without investigation, upon certificates of public officials and others and upon affidavits, certificates and written statements of officers of the Company. We have also examined and relied upon originals or copies, certified or otherwise identified to our satisfaction, of the following:

1. The Amended and Restated Certificate of Incorporation of the Company;

2. The Amended and Restated Bylaws of the Company (including all amendments thereto);

3. Records of proceedings and actions of the Board of Directors, committees thereof and stockholders of the Company relating to the Plans;

4. The Plans; and

5. Such other instruments, documents, statements and records of the Company and others as we have deemed relevant and necessary to examine and rely upon for the purpose of this opinion.

In connection with this opinion, we have assumed the legal capacity of all natural persons, the accuracy and completeness of all documents and records that we have reviewed, the genuineness of all signatures, the due authority of the parties signing such documents, the authenticity of the documents submitted to us as originals and the conformity to authentic original documents of all documents submitted to us as certified, conformed or reproduced copies. We have also assumed that, at the time of issuance of the Shares, (i) any and all agreements related to the issuance of the Shares under either Plan will conform to the terms and conditions of such Plan, as applicable, and will have been duly executed and delivered by the Company and, as applicable, the recipients of the Shares, (ii) the Committee (as defined in the applicable Plan) will have approved the issuance of awards of the Shares pursuant to the applicable Plan, and (iii) any cash consideration payable to the Company in connection with any issuance of the Shares will not be less than the par value per share of the Common Stock.

Based upon and subject to the foregoing, it is our opinion that the Shares have been duly authorized by all necessary corporate action of the Company, and when (i) the Shares are issued and delivered by the Company and (to the extent applicable) paid for in accordance with the terms of the applicable Plan and the relevant award agreements, and (ii) certificates representing the Shares in the form of the specimen certificates examined by us have been manually signed by an authorized officer of the transfer agent and registrar for the Common Stock or registered and issued electronically by such transfer agent and registrar for the Common Stock, the Shares will be validly issued, fully paid and nonassessable.

Our opinion expressed above is limited to the General Corporation Law of the State of Delaware, and we do not express any opinion concerning any other laws. This opinion is given as of the date hereof and we assume no obligation to advise you of changes that may hereafter be brought to our attention.

We hereby consent to the filing of this opinion with the Securities and Exchange Commission as an exhibit to the Registration Statement. In giving this consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Securities and Exchange Commission.

Very truly yours,

/s/ Katten Muchin Rosenman LLP

KATTEN MUCHIN ROSENMAN LLP