Exhibit 5.1

Simpson Thacher & Bartlett LLP

425 LEXINGTON AVENUE

NEW YORK, NY 10017-3954

TELEPHONE: +1-212-455-2000

FACSIMILE: +1-212-455-2502

July 2, 2018

KKR & Co. Inc.

9 West 57th Street

Suite 4200

New York, New York 10019

Ladies and Gentlemen:

We have acted as counsel to KKR & Co. Inc.(f/k/a KKR & Co. L.P.), a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-1 (File No. 333-169433), as amended by Post-Effective Amendment No. 1 on Form S-1 and Post-Effective Amendment No. 2 on Form S-3 and as further amended by Post-Effective Amendment No. 3 on Form S-3 (as amended, the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”), under the Securities Act of 1933, as amended (the “Securities Act”), relating to (1) the issuance of up to 478,105,194 shares of Class A common stock of the Company, par value $0.01 (the “Shares”), issuable upon the exchange from time to time of partnership units in each of KKR Management Holdings L.P. and KKR Fund Holdings L.P. (one Class A partner interest in each such limited partnership, collectively, a “KKR Group Partnership Unit”) and (2) the sale by KKR Holdings L.P. of certain of such Shares from time to time. As of the date hereof, 304,107,762 Shares remain issuable under the Registration Statement by the Company upon exchanges of KKR Group Partnership Units from time to time as set forth in the Registration Statement, any amendment thereto, the prospectus contained therein (the “Prospectus”) and supplements to the Prospectus, pursuant to Rule 415 under the Securities Act.

We have examined the Registration Statement, the Second Amended and Restated Exchange Agreement, dated as of May 3, 2018, among KKR Management Holdings L.P., KKR Fund Holdings L.P., KKR International Holdings L.P., KKR Holdings L.P., KKR & Co. L.P., KKR Group Holdings L.P., KKR Subsidiary Partnership L.P., KKR Group Limited and KKR Group Holdings Corp. (the “Exchange Agreement”), the Certificate of Incorporation of the Company and the Bylaws of the Company, which have been filed with the Commission as exhibits to the Registration Statement. We also have examined the originals, or duplicates or certified or conformed copies, of such other records, agreements, documents and other instruments and have made such other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that when the Shares are issued and exchanged for KKR Group Partnership Units in accordance with the Exchange Agreement as described in the Registration Statement, such Shares will be validly issued, fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the Delaware General Corporation Law.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to Post-Effective Amendment No. 3 on Form S-3 to the Registration Statement and to the use of our name under the caption “Legal Matters” in the Prospectus included in the Registration Statement.

Very truly yours,

/s/ Simpson Thacher & Bartlett LLP

SIMPSON THACHER & BARTLETT LLP