UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Dynegy Inc.

(Name of Issuer)

Common Shares, $0.01 Par Value

(Title of Class of Securities)

26817R108

(CUSIP Number)

James J. Moloney

Gibson, Dunn & Crutcher LLP

3161 Michelson Drive

Irvine, CA 92612

(949) 451-4343

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

March 4, 2015

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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Dynegy Inc.

601 Travis, Suite 1400, Houston, Texas

(A) LUMINUS MANAGEMENT, LLC

Luminus Management, LLC is the investment adviser of Luminus Energy Partners Master Fund, Ltd., Luminus Special Opportunities I Onshore, L.P. and Luminus Special Opportunities I PIE Master, L.P.

(B) LUMINUS ENERGY PARTNERS MASTER FUND, LTD.

(C) LUMINUS SPECIAL OPPORTUNITIES I ONSHORE, L.P.

(D) LUMINUS SPECIAL OPPORTUNITIES I PIE MASTER, L.P.

(E) VEGA ENERGY GP, LLC

Vega Energy GP, LLC is the general partner of Vega Asset Partners, L.P.

(F) VEGA ASSET PARTNERS, L.P.

(A) 1700 Broadway, 38th Floor, New York, NY 10019

(B) 1700 Broadway, 38th Floor, New York, NY 10019

(C) 1700 Broadway, 38th Floor, New York, NY 10019

(D) 1700 Broadway, 38th Floor, New York, NY 10019

(E) 1700 Broadway, 35th Floor, New York, NY 10019

(F) 1700 Broadway, 35th Floor, New York, NY 10019

(A) DELAWARE

(B) BERMUDA

(C) DELAWARE

(D) CAYMAN ISLANDS

(E) DELAWARE

(F) DELAWARE

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Not applicable

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Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a), (b), and (c)

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following  ¨.

Not applicable.

Not applicable.

Given the relationship between each of the Reporting Persons on this Schedule 13G, such persons may be deemed members of a group pursuant to Rule 13d-5 under the Securities Exchange Act of 1934.

Not applicable.

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By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Signature

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: March 16, 2015

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ATTENTION

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Exhibit A Schedule 13G Joint Filing Agreement

The undersigned and each other person executing this joint filing agreement (this “Agreement”) agree as follows:

(i) The undersigned and each other person executing this Agreement are individually eligible to use the Schedule 13G to which this Exhibit is attached and such Schedule 13G is filed on behalf of the undersigned and each other person executing this Agreement; and

(ii) The undersigned and each other person executing this Agreement are responsible for the timely filing of such Schedule 13G and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of the undersigned or any other person executing this Agreement is responsible for the completeness or accuracy of the information statement concerning any other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.

This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all of which, taken together, shall constitute one and the same instrument.

Dated: March 16, 2015

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Exhibit B Power of Attorney

KNOW ALL PERSONS BY THESE PRESENTS, that the undersigned Jonathan Barrett and Paul Segal, hereby each constitute and appoint Jonathan Barrett and Paul Segal, and each of them, as applicable, as true and lawful attorney-in-fact and agent, with full power of substitution and resubstitution in name, place and stead, to sign any reports on Form 3 (Initial Statement of Beneficial Ownership of Securities), Form 4 (Statement of Changes in Beneficial Ownership of Securities) and Form 5 (Annual Statement of Beneficial Ownership of Securities) relating to transactions by the undersigned in Common Shares or other securities and all amendments thereto, and all filings on Schedule 13D or Schedule 13G, and all amendments thereto, and to file the same, with the Securities and Exchange Commission and the appropriate securities exchange, granting unto said attorneys-in-fact and agents, and each of them, or their substitutes, full power and authority to do and perform each and every act and thing requisite or necessary to be done in and about the premises, as fully to all intents and purposes as the undersigned might or could do in person, hereby ratifying and confirming all that said attorneys-in-fact and agents, and each of them, or their substitutes, may lawfully do or cause to be done by virtue hereof. This Power of Attorney shall be effective until such time as the undersigned delivers a written revocation thereof to the above-named attorneys-in-fact and agents.

The undersigned each acknowledge that the foregoing attorneys-in-fact, in serving in such capacity at the request of the undersigned, are not assuming any of the undersigned’s responsibilities to comply with Section 16 of the Securities Exchange Act of 1934.

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