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As filed with the Securities and Exchange Commission on January 11, 2019

Registration No. 333-228156

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Amendment No. 2
to

FORM S-4
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

Antero Midstream GP LP
to be converted as described herein into a corporation named

Antero Midstream Corporation

(Exact name of Registrant as specified in its charter)

1615 Wynkoop Street
Denver, Colorado 80202
(303) 357-7310

(Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)

Glen C. Warren, Jr.
1615 Wynkoop Street
Denver, Colorado 80202
(303) 357-7310

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Douglas E. McWilliams
Lande A. Spottswood
Vinson & Elkins L.L.P.
1001 Fannin Street, Suite 2500
Houston, Texas 77002
(713) 758-2222

           Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement becomes effective and all other conditions to the proposed transactions contemplated by the Simplification Agreement, dated October 9, 2018, described in the enclosed joint proxy statement/prospectus, have been satisfied or waived.

           If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.    o

           If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

           If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

           Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the Exchange Act.

           If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ý

           If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

           Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer) o

           Exchange Act Rule 14d-1(d) (Cross-Border Third-Party Tender Offer) o

CALCULATION OF REGISTRATION FEE

(1)

Represents the maximum number of shares of common stock, par value $0.01 per share, of Antero Midstream Corporation ("New AM") estimated to be issuable upon the completion of the Conversion (as described herein) and the Merger (as described herein).

(2)

The proposed maximum aggregate offering price was calculated based upon the market value of the common shares of Antero Midstream GP LP ("AMGP" and, such common shares, the "AMGP Common Shares"), the securities to be converted into common stock of New AM in connection with the Conversion, and common units of Antero Midstream Partners LP ("Antero Midstream" and, such common units, the "AM Common Units"), the securities to be converted into the right to receive the Merger Consideration (as described herein) in the Merger, in accordance with Rules 457(c) and 457(f) under the Securities Act of 1933, as amended as follows: the sum of (i) the product of (a) $15.87, the average of the high and low prices of the AMGP Common Shares as reported on the New York Stock Exchange (the "NYSE") on October 30, 2018 and (b) 186,235,845, the estimated maximum number of AMGP Common Shares that will be converted into common stock of New AM in connection with the Conversion, and (ii) the product of (a) $29.73, the average of the high and low prices of the AM Common Units as reported on the NYSE on November 1, 2018 and (b) 187,330,147, the estimated maximum number of AM Common Units that may be exchanged for the Merger Consideration in the Merger.

(3)

The registrant previously paid $1,033,156 of the total registration fee in connection with the previous filings of this Registration Statement.

           This Registration Statement shall hereafter become effective in accordance with the provisions of Section 8(a) of the Securities Act of 1933.

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The information in this preliminary joint proxy statement/prospectus is not complete and may be changed. Antero Midstream Corporation may not distribute or issue the securities being registered pursuant to this registration statement until the registration statement, as filed with the Securities and Exchange Commission (of which this preliminary joint proxy statement/prospectus is a part), is effective. This preliminary joint proxy statement/prospectus is not an offer to sell nor should it be considered a solicitation of an offer to buy the securities described herein in any state where the offer or sale is not permitted.

PRELIMINARY—SUBJECT TO COMPLETION DATED JANUARY 11, 2019

CONVERSION AND MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT

            On October 9, 2018, Antero Midstream GP LP ("AMGP"), Antero Midstream Partners LP ("Antero Midstream") and certain of their affiliates entered into a Simplification Agreement, as may be amended from time to time (the "Simplification Agreement"), pursuant to which, among other things, (1) AMGP will be converted from a limited partnership to a corporation under the laws of the State of Delaware, to be named Antero Midstream Corporation (which we refer to as "New AM" and the conversion, the "Conversion"); (2) an indirect, wholly owned subsidiary of New AM will be merged with and into Antero Midstream, with Antero Midstream surviving the merger as an indirect, wholly owned subsidiary of New AM (the "Merger") and (3) all the issued and outstanding Series B Units representing limited liability company interests of Antero IDR Holdings LLC ("IDR Holdings"), a partially owned subsidiary of AMGP and the holder of all of Antero Midstream's incentive distribution rights, will be exchanged for an aggregate of approximately 17.35 million shares of New AM Common Stock (as defined below) (the "Series B Exchange"). The Conversion, the Merger, the Series B Exchange and the other transactions contemplated by the Simplification Agreement are collectively referred to as the "Transactions." As a result of the Transactions, Antero Midstream will be an indirect, wholly owned subsidiary of New AM, and former AMGP Shareholders (as defined herein), AM Unitholders (as defined herein) and holders of Series B Units will each own shares of New AM's common stock, par value $0.01 per share (the "New AM Common Stock").

            The board of directors (the "AM Board") of Antero Midstream's general partner, Antero Midstream Partners GP LLC ("AMP GP"), and the conflicts committee of the AM Board (the "AM Conflicts Committee") each has determined in good faith that the Transactions are in the best interests of Antero Midstream and the Disinterested AM Unitholders (as defined below), and have unanimously approved the Simplification Agreement and the Transactions. The board of directors of AMGP's general partner (the "AMGP Board") and the conflicts committee of the AMGP Board (the "AMGP Conflicts Committee") each has determined in good faith that the Transactions are in the best interests of AMGP and the Disinterested AMGP Shareholders (as defined below), and have unanimously approved the Simplification Agreement and the Transactions.

            If the Transactions are completed, in exchange for each common unit representing a limited partner interest in Antero Midstream (each an "AM Common Unit") held, each AM Unitholder other than Antero Resources Corporation ("Antero Resources") (collectively, the "AM Public Unitholders") will be entitled to receive, at its election and subject to proration, one of (i) $3.415 in cash without interest and 1.6350 shares of New AM Common Stock (the "Public Mixed Consideration"), (ii) 1.6350 shares of New AM Common Stock plus an additional number of shares of New AM Common Stock equal to the quotient of (A) $3.415 and (B) the average of the 20-day volume-weighted average trading price per common share representing limited partner interests in AMGP (each, an "AMGP Common Share") prior to the Election Deadline (as defined herein) (the "AMGP VWAP"), or (iii) $3.415 in cash plus an additional amount of cash equal to the product of (A) 1.6350 and (B) the AMGP VWAP. In exchange for each AM Common Unit held, Antero Resources will be entitled to receive, subject to certain adjustments, $3.00 in cash without interest and 1.6023 shares of New AM Common Stock (the "AR Mixed Consideration"). The aggregate cash consideration to be paid to Antero Resources and the AM Public Unitholders will be fixed at an amount equal to the aggregate amount of cash that would be paid if all AM Public Unitholders received $3.415 in cash per AM Common Unit (the "Public Available Cash") and Antero Resources received $3.00 in cash per AM Common Unit it owns, which is approximately $598 million in the aggregate. If the Public Available Cash exceeds the cash consideration elected to be received by the AM Public Unitholders (the amount of such excess, the "Excess Available Cash"), Antero Resources may elect to increase the total amount of cash it receives as a part of the AR Mixed Consideration up to an amount equal to the Excess Available Cash and the amount of shares it will receive will be reduced accordingly based on the AMGP VWAP. In addition, the consideration each AM Public Unitholder will receive may be prorated in the event that more cash or equity is elected to be received than what would otherwise have been paid if all AM Public Unitholders had received the Public Mixed Consideration and Antero Resources had received the AR Mixed Consideration. Based on the closing price of AMGP Common Shares on                    , 2019, the most recent trading day prior to the date of this joint proxy statement/prospectus, the aggregate value of the Merger Consideration (as defined herein) was approximately $             billion.

            AMGP and Antero Midstream will hold special meetings of their shareholders and unitholders, respectively, in connection with the proposed Transactions. At the special meeting of AMGP Shareholders (the "AMGP Special Meeting"), the AMGP Shareholders will be asked to vote on (i) a proposal to approve the Conversion, including a certificate of conversion and a certificate of incorporation (the "Conversion Proposal"), (ii) a proposal to approve the Simplification Agreement, the Merger and the other transactions contemplated in the Simplification Agreement, including the Series B Exchange (the "AMGP Transaction Proposal"), (iii) a proposal to approve the adoption of a New AM omnibus equity incentive plan (the "LTIP Proposal") and (iv) a proposal to approve the issuance of New AM Common Stock to AM Unitholders and Series B Holders pursuant to the Simplification Agreement (the "Issuance Proposal" and, together with the Conversion Proposal and the AMGP Transaction Proposal, the "AMGP Proposals"). Completion of the Transactions is not conditioned upon approval of the LTIP Proposal. Approval of the Conversion Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares. Approval of the AMGP Transaction Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares held by the AMGP Shareholders other than AMGP's general partner and its affiliates (the "Disinterested AMGP Shareholders"). Approval of both the LTIP Proposal and the Issuance Proposal requires the affirmative vote of holders of a majority of the AMGP Common Shares voted on such proposals at the AMGP Special Meeting. At the AM Special Meeting, the AM Unitholders will be asked to approve the (i) the Simplification Agreement, (ii) the Merger, and (iii) the Transactions (the "AM Merger Proposal"). Approval of the AM Merger Proposal requires the affirmative vote of holders of a majority of (i) the outstanding AM Common Units and (ii) the outstanding AM Common Units held by AM Unitholders other than Antero Midstream's general partner and its affiliates (the "Disinterested AM Unitholders").

            We cannot complete the Transactions unless the requisite AMGP Shareholders approve the AMGP Proposals and the requisite AM Unitholders approve the AM Merger Proposal. Accordingly, your vote is very important regardless of the number of AMGP Common Shares or AM Common Units you own. Voting instructions are set forth inside this joint proxy statement/prospectus.

            In light of the fact that certain affiliates of AMGP have interests in the Transactions that are different from other AMGP Shareholders, the AMGP Board formed a conflicts committee, consisting of directors that satisfy the requirements to serve on a conflicts committee set forth in the Agreement of Limited Partnership of AMGP. The AMGP Conflicts Committee and the AMGP Board each recommends that the AMGP Shareholders vote FOR each of the AMGP Proposals. The AMGP Shareholders should be aware that some of the directors and executive officers of AMGP's general partner may have interests in the Transactions that are different from, or in addition to, the interests they may have as AMGP Shareholders.

            In light of the fact that certain affiliates of Antero Midstream have interests in the Transactions that are different from other AM Unitholders, the AM Board formed a conflicts committee, consisting of directors that satisfy the requirements to serve on a conflicts committee in the Agreement of Limited Partnership of Antero Midstream. The AM Conflicts Committee and the AM Board each recommends that the AM Unitholders vote FOR the AM Merger Proposal. The AM Unitholders should be aware that some of the directors and executive officers of Antero Midstream's general partner may have interests in the Transactions that are different from, or in addition to, the interests they may have as AM Unitholders.

            This joint proxy statement/prospectus provides you with detailed information about the proposed Transactions and related matters. You are encouraged to read the entire document carefully. In particular, see "Risk Factors" beginning on page 195 of this joint proxy statement/prospectus for a discussion of risks relevant to the Transactions and New AM's business following the Transactions. See "Special Factors—Interests of Certain Persons in the Transactions" for a discussion of the interests of certain directors and executive officers of AMGP GP LLC, the general partner of AMGP, and AMP GP that are different from, or in addition to, the interests of AMGP Shareholders and AM Unitholders, respectively.

            The AMGP Common Shares are listed on the New York Stock Exchange ("NYSE") under the symbol "AMGP," and the AM Common Units are listed on the NYSE under the symbol "AM." The last reported sales price of the AMGP Common Shares on the NYSE on January 10, 2019 was $12.58. The last reported sales price of the AM Common Units on the NYSE on January 10, 2019 was $23.70.

            Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities to be issued under this joint proxy statement/prospectus or has determined if this document is truthful or complete. Any representation to the contrary is a criminal offense.

            This joint proxy statement/prospectus is dated                        , 2019 and is being first mailed to AMGP Shareholders and AM Unitholders on or about                        , 2019.

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Denver, Colorado
            , 2019

ANTERO MIDSTREAM GP LP
1615 Wynkoop Street
Denver, Colorado 80202

NOTICE OF SPECIAL MEETING OF COMMON SHAREHOLDERS

To the common shareholders of Antero Midstream GP LP:

        A special meeting (the "AMGP Special Meeting") of holders of common shares ("AMGP Shareholders") representing limited partner interests (the "AMGP Common Shares") in Antero Midstream GP LP ("AMGP") will be held on March 8, 2019 at 9:00 A.M., local time, at 1615 Wynkoop Street, Denver, Colorado 80202, for the following purposes:

•

to consider and vote on a proposal (the "Conversion Proposal") to approve the conversion (the "Conversion") of AMGP from a Delaware limited partnership to a Delaware corporation, to be named Antero Midstream Corporation ("New AM"), including a certificate of conversion and a certificate of incorporation, in connection with the transactions contemplated by the Simplification Agreement (such transactions, the "Transactions"), dated as of October 9, 2018, by and among AMGP, Antero Midstream Partners LP ("Antero Midstream") and certain of their affiliates, as may be amended from time to time (the "Simplification Agreement");



•

to consider and vote on a proposal to approve the Simplification Agreement, the merger of an indirect, wholly owned subsidiary of New AM with and into Antero Midstream as contemplated by the Simplification Agreement (the "Merger"), with Antero Midstream surviving such Merger as an indirect, wholly owned subsidiary of New AM, and the other transactions contemplated by the Simplification Agreement, including the Series B Exchange (as defined in the enclosed joint proxy statement/prospectus) (the "AMGP Transaction Proposal");



•

to consider and vote on a proposal to approve the adoption of a New AM omnibus equity incentive plan (the "LTIP Proposal"); and



•

to consider and vote on a proposal to approve the issuance of New AM common stock pursuant to the Simplification Agreement (the "Issuance Proposal" and, together with the Conversion Proposal and the AMGP Transaction Proposal, the "AMGP Proposals").

        Approval of the Conversion Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares. Approval of the AMGP Transaction Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares held by the AMGP Shareholders other than AMGP's general partner and its affiliates (the "Disinterested AMGP Shareholders"). Approval of the Issuance Proposal requires the affirmative vote of holders of a majority of the AMGP Common Shares voted on such proposal at the AMGP Special Meeting. We cannot complete the Transactions unless the requisite AMGP Shareholders approve the AMGP Proposals. Accordingly, your vote is very important regardless of the number of AMGP Common Shares you own.

        The board of directors of AMGP's general partner (the "AMGP Board") and the conflicts committee of the AMGP Board (the "AMGP Conflicts Committee"), which consists of two members of the AMGP Board who satisfy the requirements to serve on a conflicts committee under AMGP's partnership agreement, each has determined in good faith that the Transactions are in the best interests of AMGP and the Disinterested AMGP Shareholders. The AMGP Conflicts Committee and

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the AMGP Board each recommends that the AMGP Shareholders vote FOR the Conversion Proposal, FOR the AMGP Transaction Proposal and FOR the Issuance Proposal. For more information regarding the recommendation of the AMGP Conflicts Committee and the AMGP Board, see "Special Factors—Recommendation of the AMGP Conflicts Committee and the AMGP Board and Their Reasons for Recommending Approval of the Transactions."

        AMGP Shareholders should be aware that some of the directors and executive officers of AMGP's general partner may have interests in the Transactions that are different from, or in addition to, the interests they may have as AMGP Shareholders. See "Special Factors—Interests of Certain Persons in the Transactions."

        Approval of the LTIP Proposal requires the affirmative vote of holders of a majority of the AMGP Common Shares voted on such proposal at the AMGP Special Meeting. Completion of the Transactions is not conditioned upon approval of the LTIP Proposal. Even if the AMGP Shareholders approve the LTIP Proposal, AMGP will not adopt a New AM omnibus equity incentive plan unless the Transactions are completed.

        The AMGP Board unanimously determined that the approval of the LTIP Proposal is in the best interests of AMGP and the AMGP Shareholders. The AMGP Board unanimously approved the LTIP Proposal and recommends that the AMGP Shareholders vote FOR the LTIP Proposal.

        Only AMGP Shareholders of record at the close of business on January 11, 2019 are entitled to notice of and to vote at the AMGP Special Meeting. A list of AMGP Shareholders entitled to vote at the AMGP Special Meeting will be available for inspection at AMGP's offices in Denver, Colorado for any purpose relevant to the AMGP Special Meeting during normal business hours for a period of ten days before the meeting and at the AMGP Special Meeting. References to the AMGP Special Meeting in the enclosed joint proxy statement/prospectus are to the special meeting as may be adjourned or postponed.

        YOUR VOTE IS IMPORTANT. REGARDLESS OF WHETHER YOU EXPECT TO ATTEND THE AMGP SPECIAL MEETING, PLEASE SUBMIT YOUR PROXY IN ONE OF THE FOLLOWING WAYS:

•

If you hold your AMGP Common Shares in the name of a bank, broker or other nominee, you should follow the instructions provided by your bank, broker or other nominee when voting your AMGP Common Shares.



•

If you hold your AMGP Common Shares in your own name, you may submit your proxy by:



•

using the toll-free telephone number shown on the proxy card;



•

using the Internet website shown on the proxy card; or



•

marking, signing, dating and promptly returning the enclosed proxy card in the postage-paid envelope. It requires no postage if mailed in the United States.

        The enclosed joint proxy statement/prospectus provides a detailed description of the Transactions and the Simplification Agreement as well as a description of the AMGP Proposals and the LTIP Proposal. You are urged to read the joint proxy statement/prospectus, including any documents incorporated by reference, and the Annexes carefully and in their entirety. If you have any questions

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concerning the Transactions or the enclosed joint proxy statement/prospectus, would like additional copies or need help voting your AMGP Common Shares, please contact AMGP's proxy solicitor:

MacKenzie Partners, Inc.
1407 Broadway—27th Floor
New York, New York 10018
Toll free: (800) 322-2885
Collect: (212) 929-5500
proxy@mackenziepartners.com

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Denver, Colorado
            , 2019

Antero Midstream Partners LP
1615 Wynkoop Street
Denver, Colorado 80202

NOTICE OF SPECIAL MEETING OF COMMON UNITHOLDERS

To the common unitholders of Antero Midstream Partners LP:

        A special meeting (the "AM Special Meeting") of holders of common units ("AM Unitholders") of Antero Midstream Partners LP ("Antero Midstream") will be held on March 8, 2019 at 10:00 A.M., local time, at 1615 Wynkoop Street, Denver, Colorado 80202, to consider and vote on a proposal to approve, (i) the Simplification Agreement, dated as of October 9, 2018, by and among Antero Midstream GP LP ("AMGP"), Antero Midstream and certain of their affiliates, as may be amended from time to time (the "Simplification Agreement"), (ii) the merger of an indirect, wholly owned subsidiary of Antero Midstream Corporation (an entity resulting from the conversion of AMGP to a Delaware corporation) ("New AM") with and into Antero Midstream (the "Merger"), as contemplated by the Simplification Agreement, with Antero Midstream surviving such Merger as an indirect, wholly owned subsidiary of New AM, and (iii) the other transactions contemplated by the Simplification Agreement (such transactions, the "Transactions" and such proposal, the "AM Merger Proposal").

        Approval of the AM Merger Proposal requires the affirmative vote of holders of both a majority of the outstanding common units representing limited partner interests in Antero Midstream (each, an "AM Common Unit") and a majority of the outstanding AM Common Units held by AM Unitholders other than Antero Midstream's general partner and its affiliates (the "Disinterested AM Unitholders"). We cannot complete the Merger unless the requisite AM Unitholders approve the AM Merger Proposal. Accordingly, your vote is very important regardless of the number of AM Common Units you own.

        The board of directors of Antero Midstream's general partner (the "AM Board") and the conflicts committee of the AM Board (the "AM Conflicts Committee"), which consists of two members of the AM Board who satisfy the requirements to serve on a conflicts committee under the Antero Midstream partnership agreement, each has determined in good faith that the Transactions are in the best interests of Antero Midstream and the Disinterested AM Unitholders. The AM Conflicts Committee and the AM Board each recommends that the AM Unitholders vote FOR the AM Merger Proposal. For more information regarding the recommendation of the AM Conflicts Committee and the AM Board, see "Special Factors—Recommendation of the AM Conflicts Committee and the AM Board and Their Reasons for Recommending Approval of the Transactions."

        AM Unitholders should be aware that some of the directors and executive officers of Antero Midstream's general partner may have interests in the Transactions that are different from, or in addition to, the interests they may have as AM Unitholders. See "Special Factors—Interests of Certain Persons in the Transactions."

        Only AM Unitholders of record at the close of business on January 11, 2019 are entitled to notice of and to vote at the AM Special Meeting. A list of AM Unitholders entitled to vote at the AM Special Meeting will be available for inspection at Antero Midstream's offices in Denver, Colorado for any purpose relevant to the AM Special Meeting during normal business hours for a period of ten days

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before the meeting and at the AM Special Meeting. References to the AM Special Meeting in the enclosed joint proxy statement/prospectus are to the special meeting as may be adjourned or postponed.

        YOUR VOTE IS IMPORTANT. REGARDLESS OF WHETHER YOU EXPECT TO ATTEND THE AM SPECIAL MEETING, PLEASE SUBMIT YOUR PROXY IN ONE OF THE FOLLOWING WAYS:

•

If you hold your AM Common Units in the name of a bank, broker or other nominee, you should follow the instructions provided by your bank, broker or other nominee when voting your AM Common Units.



•

If you hold your AM Common Units in your own name, you may submit your proxy by:



•

using the toll-free telephone number shown on the proxy card;



•

using the Internet website shown on the proxy card; or



•

marking, signing, dating and promptly returning the enclosed proxy card in the postage-paid envelope. It requires no postage if mailed in the United States.

        The enclosed joint proxy statement/prospectus provides a detailed description of the Merger and the Simplification Agreement. You are urged to read the joint proxy statement/prospectus, including any documents incorporated by reference, and the Annexes carefully and in their entirety. If you have any questions concerning the Merger or the enclosed joint proxy statement/prospectus, would like additional copies or need help voting your AM Common Units, please contact Antero Midstream's proxy solicitor:

MacKenzie Partners, Inc.
1407 Broadway—27th Floor
New York, New York 10018
Toll free: (800) 322-2885
Collect: (212) 929-5500
proxy@mackenziepartners.com

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IMPORTANT NOTE ABOUT THIS JOINT PROXY STATEMENT/PROSPECTUS

        This joint proxy statement/prospectus, which forms part of a registration statement on Form S-4 filed with the Securities and Exchange Commission (the "SEC"), constitutes a proxy statement of Antero Midstream under Section 14(a) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), with respect to the solicitation of proxies for the AM Special Meeting to, among other things, approve the AM Merger Proposal.

        This joint proxy statement/prospectus also constitutes a proxy statement of AMGP under Section 14(a) of the Exchange Act with respect to the solicitation of proxies for the AMGP Special Meeting to, among other things, approve the AMGP Proposals and the LTIP Proposal, and a prospectus of AMGP under Section 5 of the Securities Act, for shares of New AM Common Stock that will be issued to AMGP Shareholders in the Conversion or paid to AM Unitholders in the Merger, in each case pursuant to the Simplification Agreement.

        As permitted under the rules of the SEC, this joint proxy statement/prospectus incorporates by reference important business and financial information about AMGP and Antero Midstream from other documents filed with the SEC that are not included in or delivered with this joint proxy statement/prospectus. See "Where You Can Find More Information" beginning on page 296 of this joint proxy statement/prospectus. You can obtain any of the documents incorporated by reference into this document from AMGP or Antero Midstream, as the case may be, or from the SEC's website at http://www.sec.gov. This information is also available to you without charge upon your request in writing or by telephone from AMGP or Antero Midstream at the following addresses and telephone numbers:

Antero Midstream GP LP
Antero Midstream Partners LP
Attention: Investor Relations
1615 Wynkoop Street
Denver, Colorado 80202
Telephone: (303) 357-7310

        Please note that copies of the documents provided to you will not include exhibits, unless the exhibits are specifically incorporated by reference into the documents or this joint proxy statement/prospectus.

        You may obtain certain of these documents at AMGP's website, http://www.anteromidstreamgp.com, and at Antero Midstream's website, http://www.anteromidstream.com. Information contained on AMGP's and Antero Midstream's websites is expressly not incorporated by reference into this joint proxy statement/prospectus.

        In order to receive timely delivery of requested documents in advance of the special meetings, your request should be received no later than March 1, 2019. If you request any documents, AMGP or Antero Midstream will mail them to you by first class mail, or another equally prompt means, after receipt of your request.

        AMGP and Antero Midstream have not authorized anyone to give any information or make any representation about the Transactions, AMGP or Antero Midstream that is different from, or in addition to, that contained in this joint proxy statement/prospectus or in any of the materials that have been incorporated by reference into this joint proxy statement/prospectus. Therefore, if anyone distributes this type of information, you should not rely on it. If you are in a jurisdiction where offers to exchange or sell, or solicitations of offers to exchange or purchase, the securities offered by this joint proxy statement/prospectus or the solicitation of proxies are unlawful, or you are a person to whom it is unlawful to direct these types of activities, then the offer presented in this joint proxy statement/prospectus does not extend to you. The information contained in this joint proxy statement/prospectus speaks only as of the date of this joint proxy statement/prospectus, or in the case of information in a document incorporated by reference, as of the date of such document, unless the information specifically indicates that another date applies. All information in this document concerning AMGP has been furnished by AMGP. All information in this document concerning Antero Midstream has been furnished by Antero Midstream.

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Defined Terms

        The following frequently-used terms have the meanings set forth below for purposes of this joint proxy statement/prospectus, unless the context otherwise indicates:

•

"AM Board" means the board of directors of AMP GP.



•

"AM Common Units" means the common units representing limited partner interests in Antero Midstream.



•

"AM Conflicts Committee" means the conflicts committee of the AM Board.



•

"AM LTIP" means the Antero Midstream Partners LP Long-Term Incentive Plan.



•

"AM Merger Proposal" means the proposal to approve the Merger, the Simplification Agreement and the other transactions contemplated by the Simplification Agreement, to be considered for a vote of the AM Unitholders and a separate vote of the Disinterested AM Unitholders at the AM Special Meeting.



•

"AM Parties" means Antero Midstream and AMP GP.



•

"AM Public Unitholders" means AM Unitholders, excluding Antero Resources.



•

"AM Special Meeting" means the special meeting of AM Unitholders to be held on March 8, 2019.



•

"AM Unitholder Approval" means the approval of the AM Merger Proposal by holders of a majority of the outstanding AM Common Units and by holders of a majority of the outstanding AM Common Units held by the Disinterested AM Unitholders.



•

"AM Unitholders" means all holders of AM Common Units.



•

"AMGP" means Antero Midstream GP LP, a Delaware limited partnership.



•

"AMGP Board" means the board of directors of AMGP GP.



•

"AMGP Common Shares" means the common shares representing limited partner interests in AMGP.



•

"AMGP Conflicts Committee" means the conflicts committee of the AMGP Board.



•

"AMGP GP" means AMGP GP LLC, a Delaware limited liability company and the general partner of AMGP.



•

"AMGP GP Sponsors" means the members of AMGP GP, including certain funds affiliated with Warburg Pincus LLC, certain funds affiliated with Yorktown Partners LLC, Paul M. Rady and Glen C. Warren, Jr.



•

"AMGP LTIP" means the Antero Midstream GP LP Long-Term Incentive Plan (as amended, restated, modified or supplemented from time to time).



•

"AMGP Parties" means AMGP GP, AMGP, IDR Holdings, Preferred Co, NewCo and Merger Sub.



•

"AMGP Partnership Agreement" means the Agreement of Limited Partnership of AMGP, dated as of May 9, 2017, as amended by Amendment No. 1, dated as of October 9, 2018.



•

"AMGP Proposals" means (i) the Conversion Proposal, (ii) the AMGP Transaction Proposal, and (iii) the Issuance Proposal.



•

"AMGP Shareholder Approval" means the approval of (i) the Conversion Proposal by holders of a majority of the AMGP Common Shares, (ii) the AMGP Transaction Proposal by Disinterested

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AMGP Shareholder Approval and (iii) the Issuance Proposal by a majority of the votes cast on such proposal at the AMGP Special Meeting.

•

"AMGP Shareholders" means all holders of AMGP Common Shares.



•

"AMGP Special Meeting" means the special meeting of the AMGP Shareholders to be held on March 8, 2019.



•

"AMGP Transaction Proposal" means the proposal to approve the Merger, the Simplification Agreement and the other transactions contemplated in the Simplification Agreement, including the Series B Exchange, to be considered for a vote of the requisite AMGP Shareholders at the AMGP Special Meeting.



•

"AMGP Voting Agreement" means the voting agreement between Antero Midstream and the AMGP Shareholders party thereto.



•

"AMGP VWAP" the average of the 20-day volume weighted average price per AMGP Common Share prior to the Election Deadline.



•

"AMP GP" means Antero Midstream Partners GP LLC, a Delaware limited liability company and the general partner of Antero Midstream.



•

"Antero Management" means the members of management of each of Antero Resources, AMGP GP and AMP GP, as applicable.



•

"Antero Midstream" means Antero Midstream Partners LP, a Delaware limited partnership, and its subsidiaries collectively or, if the context requires, Antero Midstream Partners LP individually.



•

"Antero Midstream Partnership Agreement" means the Agreement of Limited Partnership of Antero Midstream, dated as of November 10, 2014, as amended by Amendment No. 1, dated as of February 23, 2016, and Amendment No. 2, dated as of December 20, 2017.



•

"Antero Resources" means Antero Resources Corporation, a Delaware corporation and the holder of a majority of the outstanding AM Common Units, and its subsidiaries collectively or, if the context requires, Antero Resources Corporation individually.



•

"AR Board" means the board of directors of Antero Resources Corporation.



•

"AR Special Committee" means the special committee of the AR Board.



•

"AR Voting Agreement" means the voting agreement between AMGP and Antero Resources.



•

"Baird" means Robert W. Baird & Co. Incorporated, as financial advisor to the AR Special Committee.



•

"Code" means the Internal Revenue Code of 1986, as amended.



•

"Conversion" means the conversion of AMGP into a Delaware corporation to be named Antero Midstream Corporation (referred in this joint proxy statement/prospectus as "New AM") in accordance with the Plan of Conversion as set forth in the Simplification Agreement.



•

"Conversion Proposal" means the proposal to approve the Conversion, including a certificate of conversion and certificate of incorporation, the AMGP Shareholders will be asked to vote on.



•

"Disinterested AM Unitholder Approval" means the approval by a vote of holders of a majority of the outstanding AM Common Units held by the Disinterested AM Unitholders.



•

"Disinterested AM Unitholders" means the AM Unitholders other than AMP GP or its affiliates, which affiliates include (i) Antero Resources, (ii) the Sponsor Holders, (iii) the

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Management Holders and all other Series B Holders, (iv) the respective controlled affiliates of the persons described in the foregoing clauses (i)-(iii) and (v) any other affiliates of AMP GP identified by the AM Conflicts Committee that own AM Common Units as of the record date for the AM Special Meeting.

•

"Disinterested AMGP Shareholder Approval" means the approval by a vote of holders of a majority of the AMGP Common Shares held by the Disinterested AMGP Shareholders.



•

"Disinterested AMGP Shareholders" means the AMGP Shareholders other than AMGP GP or its affiliates, which affiliates include (i) the Sponsor Holders, (ii) the Management Holders and all other Series B Holders, (iii) the respective controlled affiliates of the Persons described in the foregoing clauses (i)-(ii) and (iv) any other affiliates of AMGP GP identified by the AMGP Conflicts Committee that own AMGP Common Shares as of the record date for the AMGP Special Meeting.



•

"Election Deadline" means, with respect to an AM Public Unitholder's election to receive Public Mixed Consideration, Public Stock Consideration or Public Cash Consideration (each as defined in this joint proxy statement/prospectus), the date by which such AM Public Unitholder must make such election with respect to such holder's AM Common Units. The Election Deadline will be 5:00 P.M., New York City time, on the later of the twentieth day following the mailing of the form of election and ten days prior to the anticipated closing date of the Transactions, or such other time as mutually agreed by AMGP and Antero Midstream.



•

"Exchange Act" means the Securities Exchange Act of 1934, as amended.



•

"Goldman Sachs" means Goldman Sachs & Co. LLC, as financial advisor to the AMGP Conflicts Committee.



•

"GP Merger" means the merger of AMP GP with and into AMGP, whereby AMGP will continue as the surviving entity and the separate existence of AMP GP will cease, which merger will, at the election of AMP GP, be completed prior to the Conversion.



•

"IDR Holdings" means Antero IDR Holdings LLC, a Delaware limited liability company and the holder of the incentive distribution rights in Antero Midstream.



•

"IDR Holdings LLC Agreement" means the limited liability company agreement of IDR Holdings, dated as of December 31, 2016, as amended by Amendment No. 1 thereto, dated as of May 9, 2018, and as amended by Amendment No. 2 thereto, dated as of October 9, 2018.



•

"Issuance Proposal" means the proposal to approve the issuance of New AM Common Stock pursuant to the Simplification Agreement.



•

"J.P. Morgan" means J.P. Morgan Securities LLC, financial advisor to Antero Resources.



•

"LTIP Proposal" means the proposal to approve the adoption of the New AM LTIP.



•

"Management Holders" means (i) Paul M. Rady, Mockingbird Investments LLC and each other entity, trust or estate planning vehicle over which Paul M. Rady controls or is deemed to have both voting and dispositive power and (ii) Glen C. Warren, Jr., Canton Investment Holdings LLC and each other entity, trust or estate planning vehicle over which Glen C. Warren, Jr. controls or is deemed to have both voting and dispositive power.



•

"Merger" means, as contemplated by the Simplification Agreement, the proposed merger of Merger Sub with and into Antero Midstream, with Antero Midstream surviving the Merger as an indirect, wholly owned subsidiary of New AM, and each AM Common Unit outstanding at the effective time of the Merger being converted into the right to receive the applicable Merger Consideration.

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•

"Merger Consideration" means, if the proposed Merger is consummated, (i) the right of each AM Public Unitholder to receive, in exchange for each AM Common Unit held by such holder, at the election of such holder and subject to proration, one of (A) the Public Mixed Consideration, (B) the Public Stock Consideration or (C) the Public Cash Consideration (each as defined herein) and (ii) the right of Antero Resources to receive, in exchange for each AM Common Unit held by it, the AR Mixed Consideration (as defined herein). Antero Resources will be able to elect to take a larger portion of the AR Mixed Consideration in cash if the AM Public Unitholders elect to receive less in cash consideration than the amount of Public Available Cash (as defined herein).



•

"Merger Sub" means Arkrose Midstream Merger Sub LLC, a Delaware limited liability company and wholly owned subsidiary of NewCo.



•

"Morgan Stanley" means Morgan Stanley & Co. LLC, financial advisor to Antero Midstream.



•

"New AM" means Antero Midstream Corporation, a Delaware corporation that will result from the Conversion.



•

"New AM Board" means the board of directors of New AM.



•

"New AM Common Stock" means the common stock of New AM, par value $0.01.



•

"New AM LTIP" means the proposed Long-Term Incentive Plan of Antero Midstream Corporation.



•

"New AM Stockholders" means the holders of New AM Common Stock.



•

"NewCo" means Arkrose Midstream NewCo Inc., a Delaware corporation and wholly owned subsidiary of AMGP.



•

"NYSE" means the New York Stock Exchange.



•

"Preferred Co" means Arkrose Midstream Preferred Co LLC, a Delaware limited liability company and wholly owned subsidiary of AMGP.



•

"SEC" means the United States Securities and Exchange Commission.



•

"Securities Act" means the Securities Act of 1933, as amended.



•

"Series A Units" means the Series A Units representing limited liability company interests in IDR Holdings.



•

"Series B Exchange Shares" means the shares of New AM Common Stock transferred in exchange for outstanding Series B Units pursuant to the Series B Exchange.



•

"Series B Holders" means the holders of the Series B Units, solely in their capacity as such.



•

"Series B Units" means the Series B Units representing limited liability company interests in IDR Holdings.



•

"Simplification Agreement" means that certain Simplification Agreement, dated as of October 9, 2018, by and among AMGP, AMGP GP, Antero Midstream, AMP GP, NewCo, Preferred Co and Merger Sub, as it may be amended from time to time, according to which the parties thereto have agreed to consummate the Transactions.



•

"Sponsor Holders" (i) Warburg Pincus Private Equity VIII, L.P., a Delaware limited partnership, Warburg Pincus Netherlands Private Equity VIII C.V. I, a company formed under the laws of the Netherlands, WP-WPVIII Investors, L.P., a Delaware limited partnership, Warburg Pincus Private Equity X O&G, L.P., a Delaware limited partnership, Warburg Pincus X Partners, L.P., a Delaware limited partnership, WP-WPVIII Investors GP L.P., a Delaware limited partnership,

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Warburg Pincus X, L.P., a Delaware limited partnership, Warburg Pincus X GP L.P., a Delaware limited partnership, WPP GP LLC, a Delaware limited liability company, Warburg Pincus Partners, L.P., a Delaware limited partnership, Warburg Pincus Partners GP LLC, a Delaware limited liability company, Warburg Pincus & Co., a New York general partnership, Warburg Pincus LLC, a New York limited liability company, and Charles R. Kaye and Joseph P. Landy, who are the Managing General Partners of Warburg Pincus & Co. and Managing Members and Co-Chief Executive Officers of Warburg Pincus LLC and (ii) Yorktown Energy Partners V, L.P., Yorktown Energy Partners VI, L.P., Yorktown Energy Partners VII, L.P. and Yorktown Energy Partners VIII, L.P.

•

"Transactions" means the transactions contemplated by the Simplification Agreement, including the GP Merger, the Conversion, the Merger, the Series B Exchange and the issuance of New AM Common Stock.



•

"Tudor Pickering" means Tudor Pickering Holt & Co Advisors LP, as financial advisor to the AM Conflicts Committee.

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QUESTIONS AND ANSWERS ABOUT THE TRANSACTIONS AND THE SPECIAL MEETINGS

        Important Information and Risks. The following are brief answers to some questions that you may have regarding the proposed Transactions, the AMGP Special Meeting and the AM Special Meeting. You should read and consider carefully the remainder of this joint proxy statement/prospectus, including the Risk Factors beginning on page 195 and the attached Annexes, because the information in this section does not provide all of the information that might be important to you. Additional important information and descriptions of risk factors are also contained in the documents incorporated by reference in this joint proxy statement/prospectus. See "Where You Can Find More Information" beginning on page 296 of this joint proxy statement/prospectus.

Q:    What is the proposed transaction and why am I receiving these materials?

A:

AMGP and Antero Midstream have agreed to a transaction by which (i) AMGP will convert from a Delaware limited partnership to a Delaware corporation named "Antero Midstream Corporation" that we refer to as "New AM," (ii) New AM and Antero Midstream will combine by merging an indirect, wholly owned subsidiary of New AM with and into Antero Midstream, with Antero Midstream surviving the Merger as an indirect, wholly owned subsidiary of New AM, and (iii) all of the issued and outstanding Series B Units of IDR Holdings, the holder of all of Antero Midstream's incentive distribution rights, will be exchanged for an aggregate of approximately 17.35 million shares of New AM Common Stock. The terms of the Simplification Agreement are described in this joint proxy statement/prospectus, and a copy of the Simplification Agreement is attached as Annex A. You are receiving these materials because the Transactions cannot be completed without certain approvals of the AMGP Shareholders and the AM Unitholders. You should carefully read this joint proxy statement/prospectus in its entirety, including any documents incorporated by reference, and the Annexes, before voting on any of the proposals.

Q:    Why are AMGP and Antero Midstream proposing the Transactions?

A:

AMGP and Antero Midstream believe that the Transactions will benefit both AMGP Shareholders and AM Unitholders for various reasons. For a discussion of those reasons, please see "Special Factors—Recommendation of the AMGP Conflicts Committee and the AMGP Board and Their Reasons for Recommending Approval of the Transactions" and "Special Factors—Recommendation of the AM Conflicts Committee and the AM Board and their Reasons for Recommending Approval of the Transactions."

Q:    What will AMGP Shareholders receive in the Conversion?

A:

If the Conversion is completed, each AMGP Shareholder will receive one share of New AM Common Stock for each AMGP Common Share held. The number of shares of New AM Common Stock is fixed and will not be adjusted on account of any change in price of the AMGP Common Shares prior to completion of the Conversion. It is expected that the New AM Common Stock received in the Conversion by former holders of AMGP Common Shares will represent approximately 36.6% of the outstanding New AM Common Stock following completion of the Transactions.

Q:    What are the expected U.S. federal income tax consequences to an AMGP Shareholder that is a U.S. person (as defined in the Code) of the Conversion?

A.

Prior to the Conversion, AMGP is a Delaware limited partnership that has elected to be treated as a corporation for U.S. federal income tax purposes. Pursuant to the Conversion AMGP will convert from a Delaware limited partnership to a Delaware corporation, and following the Conversion AMGP will continue to be treated as the same corporation for U.S. federal income tax

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purposes. As such, a holder of AMGP Common Shares that is a U.S. person (generally defined in the Code to include the categories of beneficial owners set forth in the definition of "U.S. holder" included in the section titled "Material U.S. Federal Income Tax Consequences") will be treated as owning stock in the same corporation for U.S. federal income tax purposes both before and after the Conversion and should not recognize gain or loss for U.S. federal income tax purposes as a result of the Conversion.

Q:    What will AM Unitholders receive in the Merger?

A:

If the Merger is completed, in exchange for each AM Common Unit held, each AM Public Unitholder will be entitled to receive, at its election and subject to proration, one of (i) the Public Mixed Consideration, (ii) the Public Stock Consideration, or (iii) the Public Cash Consideration. In exchange for each AM Common Unit held, Antero Resources will be entitled to receive, subject to certain adjustments, the AR Mixed Consideration. Based on the closing price of AMGP Common Shares on                        , 2019, the most recent trading day prior to the date of this joint proxy statement/prospectus, the aggregate value of the Merger Consideration was approximately $             billion. It is expected that the New AM Common Stock received in the Merger by former holders of AM Common Units will represent approximately 60% of the outstanding New AM Common Stock following completion of the Transactions.

Q:    What are the expected U.S. federal income tax consequences to an AM Public Unitholder that is a U.S. holder (as defined in the section titled "Material U.S. Federal Income Tax Consequences") of the Merger?

A.

The Merger should be a taxable event for each AM Public Unitholder that is a U.S. holder. The specific expected U.S. federal income tax consequences of the Merger to an AM Public Unitholder that is a U.S. holder are dependent upon whether such holder receives only Public Cash Consideration or also receives Public Stock Consideration.

U.S. Holders Who Receive Only Public Cash Consideration

If a U.S. holder receives only Public Cash Consideration, the payment of the Public Cash Consideration should be treated for U.S. federal income tax purposes as a distribution from Antero Midstream to such U.S. holder in liquidation of such U.S. holder's interest in Antero Midstream that will only be taxable to the U.S. holder to the extent the amount of the distribution exceeds the U.S. holder's tax basis in its AM Common Units. The character of any such recognized gain or loss is described below.

U.S. Holders Who Receive Public Stock Consideration

If a U.S. holder receives Public Stock Consideration and Public Cash Consideration and/or cash in lieu of fractional shares of New AM Common Stock, the payment of the cash amounts should be treated for U.S. federal income tax purposes as a current distribution to such U.S. holder from Antero Midstream that will only be taxable to the U.S. holder to the extent the amount of the distribution exceeds the U.S. holder's tax basis in its AM Common Units.

The receipt of the Public Stock Consideration by a U.S. holder should be treated for U.S. federal income tax purposes as a taxable sale of AM Common Units for New AM Common Stock, with such taxable sale being treated as occurring immediately after the current distribution described above. Such U.S. holder will recognize gain or loss in an amount equal to the difference between (i) the sum of (A) the fair market value of any New AM Common Stock received and (B) such U.S. holder's share of Antero Midstream's nonrecourse liabilities immediately prior to the Merger and (ii) such U.S. holder's adjusted tax basis in the AM Common Units exchanged therefor (which

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will include such U.S. holder's share of Antero Midstream's nonrecourse liabilities immediately prior to the Merger), as reduced (but not below zero) by any Public Cash Consideration and any cash in lieu of fractional shares of New AM Common Stock received by such U.S. holder.

Gain or loss recognized by a U.S. holder in connection with the receipt of Merger Consideration will generally be capital gain or loss. However, a portion of such gain or loss, which portion could be substantial, will be separately computed and taxed as ordinary income or loss to the extent attributable to assets giving rise to depreciation recapture or other "unrealized receivables" or to "inventory items" owned by Antero Midstream and its subsidiaries. Passive losses that were not deductible by a U.S. holder in prior taxable periods because they exceeded the U.S. holder's share of Antero Midstream's income may become available to offset a portion of any gain recognized by such U.S. holder in connection with the receipt of Merger Consideration.

See the section titled "Material U.S. Federal Income Tax Consequences" for a more complete discussion of certain U.S. federal income tax consequences of the Merger.

Q:    What will happen to the Series B Units in connection with the Transactions?

A:

If the Transactions are completed, each issued and outstanding Series B Unit will be exchanged for 176.0041 shares of New AM Common Stock, or an aggregate of approximately 17.35 million shares of New AM Common Stock. A total of 98,600 Series B Units are currently outstanding. The Series B Exchange Shares will be subject to the same vesting conditions to which the Series B Units are currently subject, with two-thirds fully vested and one-third scheduled to vest at December 31, 2019. Distributions declared but not yet paid with respect to vested Series B Units will be delivered in connection with the Series B Exchange. Consistent with the existing terms of the Series B Units, distributions declared on unvested Series B Units prior to the closing of the Transactions will not be paid until the applicable vesting date of the Series B Exchange Shares. At the time of the Transactions, cash in an amount equal to these unpaid distributions will be deposited into an escrow account to be released at the time of vesting, excluding any amounts attributable to any distributions made with respect to unvested Series B Units after December 31, 2018 but prior to the effective time of the Merger. With respect to Series B Units and Series B Exchange Shares scheduled to vest on December 31, 2019, the Series B Holders have agreed to forgo any distributions from IDR Holdings and any dividends from New AM that are paid with respect to such units or shares, as applicable, during the twelve months ending December 31, 2019. Based on the number of outstanding AM Common Units and AMGP Common Shares and their respective closing sales prices as of October 8, 2018 (the last trading day before the day the Simplification Agreement was announced), it is expected that the Series B Exchange Shares will represent approximately 4.4% of the pro forma market capitalization of New AM in excess of $2 billion (representing approximately 3.4% of the total pro forma market capitalization of New AM).

All of the Series B Units are held by certain members of Antero Management. For more detail about the interests of the executive officers and directors of AMGP GP and AMP GP in the Transactions, see "Special Factors—Interests of Certain Persons in the Transactions" beginning on page 178 of this joint proxy statement/prospectus.

Q:    What will happen to AM phantom units in the Transactions?

A:

At the effective time of the Merger, each award of Antero Midstream phantom units granted pursuant to the AM LTIP (as defined below) that remains outstanding immediately prior to the effective time of the Merger, whether vested or unvested, will be converted into a restricted stock unit or similar award of New AM, with substantially the same terms and conditions (including with respect to vesting) applicable to such AM Phantom Unit Award (as defined below) immediately

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prior to the effective time of the Merger. For more information on the treatment of AM phantom units, see "Special Factors—The AMGP Proposals and the AM Merger Proposal: The Simplification Agreement and the Transactions—Treatment of Series B Units and Equity Awards Held by Antero Midstream Executives."

Q:    Where will my shares or units trade after the completion of the Transactions?

A:

New AM Common Stock will trade on the NYSE under the symbol "AM." AM Common Units will no longer be publicly traded after the completion of the Transactions.

Q:    What happens to my future distributions or dividends?

A:

If the date of the closing of the Transactions is after the record date set by the AMGP Board in connection with declared distributions to be paid by AMGP to AMGP Shareholders, then a former AM Unitholder will not receive dividends for that quarter on the New AM Common Stock it receives in the Transactions, but will receive distributions for that quarter declared by Antero Midstream prior to the closing of the Transactions, if such former AM Unitholder was a record holder of such AM Common Units as of the record date with respect to such distribution. AM Unitholders will not receive both distributions from Antero Midstream and dividends from New AM for the same quarter. See "The AMGP Proposals and the AM Merger Proposal: The Simplification Agreement and the Transactions—Dividends and Distributions."

Current AMGP Shareholders will continue to receive dividends on the New AM Common Stock they receive in the Conversion at the discretion of the New AM Board. See "The AMGP Proposals and the AM Merger Proposal: The Simplification Agreement and the Transactions—Dividends and Distributions."

New AM will not be required to pay dividends on the New AM Common Stock under the New AM certificate of incorporation or the DGCL. Subject to preferences that may be applicable to any outstanding shares or series of preferred stock, New AM Stockholders will only be entitled to receive ratably such dividends (payable in cash, stock or otherwise), if any, as may be declared from time to time by the New AM Board out of funds legally available for dividend payments. The New AM Board will make a determination each quarter as to the actual amount, if any, of dividends to pay on the New AM Common Stock, based on various factors, some of which are beyond its control, including New AM's operating cash flows, New AM's working capital needs, New AM's ability to access capital markets for debt and equity financing on reasonable terms, the restrictions contained in New AM's debt instruments, if any, New AM's debt service requirements, credit metrics and the cost of acquisitions, if any. New AM may not have sufficient cash each quarter to pay dividends or maintain current or expected levels of dividends. Accordingly, we cannot guarantee that any dividends will be declared or paid in the future.

Q:    Are there risks associated with the Transactions that I should consider in deciding how to vote?

A:

Yes. You should carefully read the detailed description of the risks associated with the Transactions, as described under the heading "Risk Factors" beginning on page 195 and "Comparison of the Rights of New AM Stockholders, AMGP Shareholders and AM Unitholders" beginning on page 243 of this joint proxy statement/prospectus.

Q:    When and where will the special meetings be held?

A:

AMGP Shareholders:    The AMGP Special Meeting will be held at 1615 Wynkoop Street, Denver, Colorado 80202, on March 8, 2019, at 9:00 A.M., local time.

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AM Unitholders:    The AM Special Meeting will be held at 1615 Wynkoop Street, Denver, Colorado 80202, on March 8, 2019, at 10:00 A.M., local time.

Q:    Who is entitled to vote at the special meetings?

A:

AMGP Shareholders:    The record date for the AMGP Special Meeting is January 11, 2019. Only AMGP Shareholders of record as of the close of business on the record date are entitled to notice of, and to vote at, the AMGP Special Meeting.

AM Unitholders:    The record date for the AM Special Meeting is January 11, 2019. Only AM Unitholders of record as of the close of business on the record date are entitled to notice of, and to vote at, the AM Special Meeting.

Q:    What constitutes a quorum at the special meetings?

A:

AMGP Shareholders:    The holders of a majority of the outstanding AMGP Common Shares, represented in person or by proxy (by submitting a properly executed proxy card or properly submitting your proxy by telephone or Internet), as of the record date will constitute a quorum and will permit AMGP to conduct the proposed business at the AMGP Special Meeting. Proxies received but marked as abstentions will be counted as AMGP Common Shares that are present and entitled to vote for purposes of determining the presence of a quorum. Broker non-votes will be counted as AMGP Common Shares that are present and entitled to vote for purposes of determining the presence of a quorum.

AM Unitholders:    The holders of a majority of the outstanding AM Common Units represented in person or by proxy (by submitting a properly executed proxy card or properly submitting a proxy by telephone or Internet) will constitute a quorum and will permit Antero Midstream to conduct the proposed business at the AM Special Meeting. Proxies received but marked as abstentions will be counted as AM Common Units that are present and entitled to vote for purposes of determining the presence of a quorum. Broker non-votes will not be considered present at the AM Special Meeting for purposes of determining the presence of a quorum and will not be included in the vote.

Q:    What is the vote required to approve each proposal?

A:

AMGP Shareholders:    Approval of the Conversion Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares. Approval of the AMGP Transaction Proposal requires the affirmative vote of holders of a majority of the outstanding AMGP Common Shares held by the Disinterested AMGP Shareholders. Approval of both the LTIP Proposal and the Issuance Proposal requires the affirmative vote of holders of a majority of the AMGP Common Shares voted on such proposals at the AMGP Special Meeting. Completion of the Transactions is not conditioned upon approval of the LTIP Proposal. Even if the AMGP Shareholders approve the LTIP Proposal, AMGP will not adopt a New AM LTIP unless the Transactions are completed.

As of January 8, 2019, all of the directors, executive officers and affiliates of AMGP GP beneficially owned, in the aggregate, approximately 58% of the outstanding AMGP Common Shares. Pursuant to the AMGP Voting Agreement, the AMGP Voting Agreement Shareholders have agreed to vote in favor of each of the AMGP Proposals. AMGP believes that all of the remaining directors and executive officers of AMGP GP will also vote in favor of each of the AMGP Proposals.

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AM Unitholders:    Approval of the AM Merger Proposal requires the affirmative vote of both the holders of a majority of the outstanding AM Common Units and a majority of the outstanding AM Common Units held by the Disinterested AM Unitholders.

As of January 8, 2019, all of the directors and executive officers of AMP GP beneficially owned, in the aggregate, less than 1% of the outstanding AM Common Units. Pursuant to the AR Voting Agreement, Antero Resources, which beneficially owns approximately 53% of the outstanding AM Common Units, has agreed to vote or cause to be voted all AM Common Units beneficially owned by it in favor of the AM Merger Proposal. Antero Midstream believes that all of the directors and executive officers of AMP GP will vote in favor of the AM Merger Proposal.

Q:    What happens if I sell my AM Common Units after the record date but before the AM Special Meeting?

A:

If you transfer your AM Common Units after the record date but before the date of the AM Special Meeting, you will retain your right to vote at the AM Special Meeting, but you will not have the right to receive the Merger Consideration. In order to receive the Merger Consideration, you must hold your AM Common Units through the completion of the Merger.

Q:    What happens if I sell my AMGP Common Shares after the record date but before the AMGP Special Meeting?

A:

If you transfer your AMGP Common Shares after the record date but before the date of the AMGP Special Meeting, you will still retain your right to vote at the AMGP Special Meeting, but you will not receive any shares of New AM Common Stock with respect to such AMGP Common Shares in connection with the Transactions.

Q:    How do I vote my AMGP Common Shares or AM Common Units if I hold them in my own name?

A:

AMGP Shareholders:    After you have read this joint proxy statement/prospectus carefully, please respond by completing, signing and dating your proxy card and returning it in the enclosed postage-paid envelope, or by submitting your proxy by telephone or the Internet as soon as possible in accordance with the instructions provided under "The AMGP Special Meeting—Proxy Submission Procedures—Proxy Submission by AMGP Shareholders of Record."

AM Unitholders:    After you have read this joint proxy statement/prospectus carefully, please respond by completing, signing and dating your proxy card and returning it in the enclosed postage-paid envelope, or by submitting your proxy by telephone or the Internet as soon as possible in accordance with the instructions provided under "The AM Special Meeting—Proxy Submission Procedures—Proxy Submission by AM Unitholders of Record."

Q:    If my AMGP Common Shares or AM Common Units are held in "street name" by my bank, broker or other nominee, will my bank, broker or other nominee vote them for me?

A:

AMGP Shareholders:    If your AMGP Common Shares are held in "street name" in a brokerage account or by a broker, bank or other nominee, you must provide the record holder of your AMGP Common Shares with instructions on how to vote your shares. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote AMGP Common Shares held in street name by returning a proxy card directly to AMGP or by voting in person at the AMGP Special Meeting unless you provide a "legal proxy," which you must obtain from your broker, bank or other nominee. Your broker, bank or other nominee is obligated to provide you with a voting instruction card for you to use.

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Under the rules of the NYSE, brokers who hold shares in street name for a beneficial owner of those shares typically have the authority to vote in their discretion on "routine" proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that the NYSE determines to be "non-routine" without specific instructions from the beneficial owner. It is expected that each of the AMGP Proposals is a "non-routine" matter. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of shares to vote on a particular proposal for which the broker does not have discretionary voting power.

If you are an AMGP Shareholder and you do not instruct your broker, bank or other nominee on how to vote your shares, your broker, bank or other nominee may not vote your shares on the AMGP Proposals, which broker non-votes, if any, will have the same effect as a vote cast "AGAINST" both the Conversion Proposal and the AMGP Transaction Proposal. A broker non-vote will have no effect on the vote count for the Issuance Proposal or the LTIP Proposal.

AM Unitholders:    If your AM Common Units are held in "street name" in a brokerage account or by a broker, bank or other nominee, you must provide the record holder of your AM Common Units with instructions on how to vote your AM Common Units. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote AM Common Units held in street name by returning a proxy card directly to Antero Midstream or by voting in person at the AM Special Meeting unless you provide a "legal proxy," which you must obtain from your broker, bank or other nominee. Your broker, bank or other nominee is obligated to provide you with a voting instruction card for you to use.

Under the rules of the NYSE, brokers who hold units in street name for a beneficial owner of those units typically have the authority to vote in their discretion on "routine" proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that the NYSE determines to be "non-routine" without specific instructions from the beneficial owner. It is expected that all proposals to be voted on at the AM Special Meeting are such "non-routine" matters. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of units to vote on a particular proposal for which the broker does not have discretionary voting power.

If you are an AM Unitholder and you do not instruct your broker, bank or other nominee on how to vote your units, your broker, bank or other nominee may not vote your units on the AM Merger Proposal, which broker non-votes, if any, will have the same effect as a vote "AGAINST" this proposal.

Q:    When do you expect the Transactions to be completed?

A:

We currently expect the Transactions to close in the first quarter of 2019. A number of conditions must be satisfied before AMGP and Antero Midstream can complete the Transactions, including the approval of the Conversion Proposal, the AMGP Transaction Proposal and the Issuance Proposal by the AMGP Shareholders and the approval of the AM Merger Proposal by the AM Unitholders. Although AMGP and Antero Midstream cannot be sure when all of the conditions to the Transactions will be satisfied, AMGP and Antero Midstream expect to complete the Transactions as soon as practicable following the AMGP Special Meeting and AM Special Meeting (assuming the AMGP Proposals and the AM Merger Proposal are approved by the AMGP Shareholders and AM Unitholders, respectively), subject to, among other things, the registration statement of which this joint proxy statement/prospectus forms a part having been declared effective under the Securities Act. See "The AMGP Proposals and the AM Merger Proposal: The Simplification Agreement and the Transactions—Conditions to the Merger" and "Risk Factors—Risks Related to the Transactions." Failure to complete the Transactions, or significant delays in

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completing the Transactions, could negatively affect each party's future business and financial results and the trading prices of the AMGP Common Shares and AM Common Units.

Q:    How do the AMGP Conflicts Committee and the AMGP Board recommend that the AMGP Shareholders vote?

A:

The AMGP Conflicts Committee and the AMGP Board each recommends that the AMGP Shareholders vote FOR each of the AMGP Proposals.

On October 8, 2018, the AMGP Conflicts Committee unanimously (i) determined in good faith that the Transactions, including the Merger and the issuance or payment of the Merger Consideration, on the terms set forth in the Simplification Agreement and the other transaction documents, are in the best interests of AMGP and the Disinterested AMGP Shareholders, (ii) approved the Transactions, including the Merger and the issuance or payment of the Merger Consideration, on the terms set forth in the Simplification Agreement and the other transaction documents, (iii) recommended to the AMGP Board that the AMGP Board approve the Transactions, including the Merger and the issuance or payment of the Merger Consideration, on the terms set forth in the Simplification Agreement and the other transaction documents, and submit the AMGP Proposals to a vote of the AMGP Shareholders and (iv) recommended to the Disinterested AMGP Shareholders that the Disinterested AMGP Shareholders approve the Transactions, including the Merger and the issuance or payment of the Merger Consideration, on the terms set forth in the Simplification Agreement and the other transaction documents. For more information regarding the recommendation of the AMGP Conflicts Committee, see "Special Factors—Recommendation of the AMGP Conflicts Committee and the AMGP Board and Their Reasons for Recommending Approval of the Transactions."

AMGP Shareholders should be aware that some of AMGP GP's directors and executive officers may have interests in the Transactions that are different from, or in addition to, the interests they may have as AMGP Shareholders. See "Special Factors—Interests of Certain Persons in the Transactions"

Q:    How do the AM Conflicts Committee and the AM Board recommend that the AM Unitholders vote?

A:

The AM Conflicts Committee and the AM Board each recommends that the AM Unitholders vote FOR the AM Merger Proposal.

On October 8, 2018, the AM Conflicts Committee unanimously (i) determined in good faith that the Transactions, including the Merger, on the terms set forth in the Simplification Agreement and the other transaction documents, are in the best interests of AM and the Disinterested AM Unitholders, (ii) approved the Transactions, including the Merger, on the terms set forth in the Simplification Agreement and the other transaction documents, (iii) recommended to the AM Board that the AM Board approve the Transactions, including the Merger, on the terms set forth in the Simplification Agreement and the other transaction documents, and submit the AM Merger Proposal to a vote of the AM Unitholders and (iv) recommended to the Disinterested AM Unitholders that the Disinterested AM Unitholders approve the Transactions, including the Merger, on the terms set forth in the Simplification Agreement and the other transaction documents. For more information regarding the recommendation of the AM Conflicts Committee in making such determination under the Antero Midstream Partnership Agreement, see "Special Factors—Recommendation of the AM Conflicts Committee and the AM Board and Their Reasons for Recommending Approval of the Transactions"

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AM Unitholders should be aware that some of AMP GP's directors and executive officers may have interests in the Simplification that are different from, or in addition to, the interests they may have as AM Unitholders. See "Special Factors—Interests of Certain Persons in the Transactions."

Q:    What will happen to Antero Midstream as a result of the Merger?

A:

At the closing of the Merger, an indirect, wholly owned subsidiary of New AM will be merged with and into Antero Midstream, with Antero Midstream surviving as an indirect, wholly owned subsidiary of New AM. Upon completion of the Merger, AM Common Units currently listed on the NYSE will cease to be listed on the NYSE and will be subsequently deregistered under the Exchange Act.

Q.    What are the expected U.S. federal income tax consequences for an AM Public Unitholder that is a U.S. holder (as defined in the section titled "Material U.S. Federal Income Tax Consequences") of the ownership of shares of New AM Common Stock after the Merger is completed?

A.

New AM will be classified as a corporation for U.S. federal income tax purposes, and thus New AM (and not its stockholders) will be subject to U.S. federal income tax on its taxable income. Distributions with respect to New AM Common Stock will constitute dividends for U.S. federal income tax purposes to the extent paid from New AM's current or accumulated earnings and profits (as determined under U.S. federal income tax principles). To the extent that the amount of a distribution exceeds New AM's current and accumulated earnings and profits, such distribution will first be treated as a non-taxable return of capital to the extent of (and reducing, but not below zero) the U.S. holder's adjusted tax basis in its New AM Common Stock and thereafter be treated as capital gain from the sale or exchange of such New AM Common Stock.

See the section titled "Material U.S. Federal Income Tax Consequences" for a more complete discussion of certain U.S. federal income tax consequences of owning and disposing of shares of New AM Common Stock.

Q:    Are AM Unitholders entitled to appraisal rights?

A:

No. AM Unitholders are not entitled to appraisal rights in connection with the Merger under applicable law or contractual appraisal rights under the Antero Midstream Partnership Agreement or the Simplification Agreement.

Q:    What if I do not vote?

A:

If you sign and return your proxy card without indicating how to vote on any particular proposal, the AMGP Common Shares represented by your proxy will be voted as recommended by the AMGP Board with respect to that proposal or the AM Common Units represented by your proxy will be voted as recommended by the AM Board with respect to that proposal. Unless an AMGP Shareholder or AM Unitholder, as applicable, checks the box on its proxy card to withhold discretionary authority, the applicable proxy holders may use their discretion to vote on other matters relating to the AMGP Special Meeting or AM Special Meeting, as applicable.

For purposes of each of the AMGP Special Meeting and the AM Special Meeting, an abstention occurs when a shareholder or unitholder, as applicable, attends the applicable special meeting in person and does not vote or returns a proxy with an "abstain" instruction.

AMGP

Conversion Proposal:    An abstention or failure to vote will have the same effect as a vote cast "AGAINST" the Conversion Proposal.

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AMGP Transaction Proposal:    An abstention or failure to vote will have the same effect as a vote cast "AGAINST" the AMGP Transaction Proposal.

LTIP Proposal:    An abstention will be deemed to not have been cast on such matter. If an AMGP Shareholder is not present in person at the AMGP Special Meeting and does not respond by proxy, it will have no effect on the vote count for the LTIP Proposal (assuming a quorum is present).

Issuance Proposal:    An abstention will be deemed to not have been cast on such matter. If an AMGP Shareholder is not present in person at the AMGP Special Meeting and does not respond by proxy, it will have no effect on the vote count for the Issuance Proposal (assuming a quorum is present).

Antero Midstream

AM Merger Proposal:    An abstention or failure to vote will have the same effect as a vote cast "AGAINST" the AM Merger Proposal.

Q:    If I am planning to attend a special meeting in person, should I still vote by proxy?

A:

Yes. Regardless of whether you plan to attend the AMGP Special Meeting or the AM Special Meeting, as applicable, you should vote by proxy. Your AMGP Common Shares or AM Common Units will not be voted if you do not vote by proxy or do not vote in person at the AMGP Special Meeting or the AM Special Meeting, as applicable.

Q:    Who may attend the AMGP Special Meeting and the AM Partners Special Meeting?

A:

AMGP Shareholders (or their authorized representatives) and AMGP's invited guests may attend the AMGP Special Meeting. AM Unitholders (or their authorized representatives) and Antero Midstream's invited guests may attend the AM Special Meeting. All attendees should be prepared to present government-issued photo identification (such as a driver's license or passport) for admittance.

Q:    Can I change my instructions after I have submitted my proxy?

A:

Yes. If you own your AMGP Common Shares or AM Common Units in your own name, you may revoke your proxy at any time prior to its exercise by:

•

giving written notice of revocation to the Secretary of AMGP GP or the Secretary of AMP GP, as applicable, at or before the AMGP Special Meeting or the AM Special Meeting, as applicable;

•

appearing and voting in person at the AMGP Special Meeting or the AM Special Meeting, as applicable; or

•

properly completing and executing a later dated proxy and delivering it to the Secretary of AMGP GP or the Secretary of AMP GP, as applicable, at or before the AMGP Special Meeting or the AM Special Meeting, as applicable.

Your presence without voting at the AMGP Special Meeting or the AM Special Meeting, as applicable, will not automatically revoke your proxy, and any revocation during the meeting will not affect votes previously taken.

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Q:    What will happen to Antero Midstream if the Merger does not occur?

A:

If the AMGP Proposals are not approved by the requisite AMGP Shareholders or the AM Merger Proposal is not approved by the requisite AM Unitholders, or if the Merger is not completed for any other reason, AM Unitholders will not receive any form of consideration for their AM Common Units in connection with the Merger. Instead, Antero Midstream will remain a public limited partnership and the AM Common Units will continue to be listed and traded on the NYSE.

Q:    How and when do I make my election with respect to the Merger Consideration?

A:

You will receive a form of election in a separate mailing. You should carefully review and follow the instructions accompanying that form of election. You will make your consideration election by properly completing, signing and returning the form of election along with your evidence of units in book-entry form representing AM Common Units to American Stock Transfer & Trust Company, LLC, the exchange agent in connection with the Merger. The Election Deadline will be 5:00 P.M., New York City time, on the later of the twentieth day following the mailing of the form of election and ten days prior to the anticipated closing date of the Transactions, or such other time as mutually agreed by AMGP and Antero Midstream. AMGP and Antero Midstream will publicly announce the anticipated Election Deadline at least five business days prior to the Election Deadline in a press release, on AMGP's and Antero Midstream's websites and in filings with the SEC. If you receive New AM Common Stock as part of the Merger Consideration, the market price of the Merger Consideration you ultimately receive will depend on the closing price of New AM Common Stock on the date the Transactions are consummated. As such, you will not know the exact market value of the Merger Consideration you will receive at the time you make your election.

Q:    Can I change my election after the form of election has been submitted?

A:

Yes, as long as you do so before the Election Deadline. You may revoke your election prior to the Election Deadline by submitting a written notice of revocation to the exchange agent or by submitting new later-dated election materials. Revocations must specify the name in which your AM Common Units are registered on the unit transfer books of Antero Midstream and such other information as the exchange agent may request. If you wish to submit a new election, you must do so in accordance with the election procedures described in this joint proxy statement/prospectus and in the form of election that you will receive in a separate mailing. If you instructed a bank, broker, nominee, trust company or other fiduciary to submit an election for your AM Common Units, you must follow the directions of your bank, broker, nominee, trust company or other fiduciary for changing those instructions. Whether you revoke your election by submitting a written notice of revocation or by submitting new election materials, the notice of materials must be received by the exchange agent by the Election Deadline in order for the revocation or new election to be valid.

Q:    May I transfer my AM Common Units after I make my election?

A:

Yes, however, any transfer prior to the Election Deadline of AM Common Units made by an AM Public Unitholder who has made an election will automatically revoke such election. AM Public Unitholders who have made elections will be unable to sell or otherwise transfer their AM Common Units after the Election Deadline, unless the election is properly revoked before the Election Deadline or unless the Simplification Agreement is terminated.

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Q:    What if I do not send a form of election or it is not received?

A:

If the exchange agent does not receive a properly completed form of election from you before the Election Deadline, together with any unit certificates (or evidence of units in book-entry form) representing the units you wish to exchange for the Merger Consideration, properly endorsed for transfer, book-entry transfer shares or a guarantee of delivery and any additional documents required by the procedures set forth in the form of election, then you will be deemed to have made a Public Mixed Consideration election with respect to your AM Common Units. You bear the risk of delivery and should send any form of election by courier or by hand delivery to the appropriate address shown in the form of election.

If you do not make a valid election with respect to any AM Common Units you own of record, you will receive written instructions from the exchange agent after completion of the proposed Transactions on how to exchange your AM Common Units for the Merger Consideration.

Q:    May I submit a form of election even if I do not vote for the approval of the Transactions?

A:

Yes. You may submit a form of election even if you vote against the approval of the Transactions or if you abstain from voting.

Q:    What should I do if I receive more than one set of voting materials for the AMGP Special Meeting or the AM Special Meeting?

A:

You may receive more than one set of voting materials for the AMGP Special Meeting or the AM Special Meeting and the materials may include multiple proxy cards. For example, you will receive a separate voting instruction card for each brokerage account in which you hold AMGP Common Shares or AM Common Units. Additionally, if you are a holder of record registered in more than one name, you will receive more than one proxy card. Finally, if you hold both AMGP Common Shares and AM Common Units, you will receive two separate packages of proxy materials. Please complete, sign, date and return each proxy card and voting instruction card that you receive according to the instructions on it.

Q:    Whom do I call if I have further questions about voting, the special meetings or the Transactions?

A:

AMGP Shareholders and AM Unitholders who have questions about the Transactions, including the procedures for voting their common shares or units, or who desire additional copies of this joint proxy statement/prospectus or additional proxy cards should contact: