Filed by: Omnicom Group Inc.

pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: The Interpublic Group of Companies, Inc.

SEC File No.: 001-06686

The following email was sent by John Wren, Chairman and Chief Executive Officer of Omnicom Group Inc., to Omnicom employees on December 9, 2024.

Subject: A major milestone for Omnicom

All,

We are proud to share that we have reached a definitive agreement to acquire Interpublic, creating the industry’s premier marketing and sales organization.

This strategic combination creates a new enterprise with unmatched capabilities across media, precision marketing, commerce, data, healthcare, public relations, and creativity. We will also have the most advanced marketing and sales platform in the industry with the ability to accelerate innovation by focusing on and amplifying technology and AI investments across a larger enterprise.

Omnicom and Interpublic have highly complementary cultures and share core values, including a foundational belief in innovation and the understanding that talent is what drives success in this industry. That will not change.

This transaction will create many opportunities as we welcome highly awarded agencies and talented colleagues, valued clients, and enhanced platforms for data-driven marketing outcomes. Together, these benefits will provide greater opportunities for growth.

Until the closing of the transaction, which is expected in the second half of 2025, we will continue to operate as independent companies. The transaction is subject to Omnicom and Interpublic shareholder approvals, required regulatory approvals, and other customary closing conditions.

In the meantime, it’s business as usual and you should remain focused on continuing to deliver great work for your clients.

You can find additional details of the transaction in Omnicom’s press release.

We hope you join us in celebrating this incredible moment and take pride in being part of a company even better positioned for the future.

Thanks,

Chairman & CEO

Omnicom

FORWARD-LOOKING STATEMENTS

This communication contains certain “forward-looking statements” within the meaning of federal securities laws. Forward-looking statements may be identified by words such as “anticipates,” “believes,” “could,” “continue,” “estimate,” “expects,” “intends,” “will,” “should,” “may,” “plan,” “predict,” “project,” “would” and similar expressions. Forward-looking statements are not statements of historical fact and reflect Omnicom’s and IPG’s current views about future events. Such forward-looking statements include, without limitation, statements about the benefits of the proposed transaction involving Omnicom and IPG, including future financial and operating results, Omnicom’s and IPG’s plans, objectives, expectations and intentions, the expected timing and likelihood of completion of the proposed transaction, and other statements that are not historical facts, including the combined company’s ability to create an advanced marketing and sales platform, the combined company’s ability to accelerate innovation and enhance efficiency through the transaction, and the combined company’s plan on future stockholder returns. No assurances can be given that the forward-looking statements contained in this communication will occur as projected, and actual results may differ materially from those projected. Forward-looking statements are based on current expectations, estimates and assumptions that involve a number of risks and uncertainties that could cause actual results to differ materially from those projected. These risks and uncertainties include, without limitation, the ability to obtain the requisite Omnicom and IPG stockholder approvals; the risk that Omnicom or IPG may be unable to obtain

governmental and regulatory approvals required for the proposed transaction (and the risk that such approvals may result in the imposition of conditions that could adversely affect the combined company or the expected benefits of the proposed transaction); the risk that an event, change or other circumstance could give rise to the termination of the proposed transaction; the risk that a condition to closing of the proposed transaction may not be satisfied; the risk of delays in completing the proposed transaction; the risk that the businesses will not be integrated successfully or that the integration will be more costly or difficult than expected; the risk that the cost savings and any other synergies from the proposed transaction may not be fully realized or may take longer to realize than expected; the risk that any announcement relating to the proposed transaction could have adverse effects on the market price of Omnicom’s or IPG’s common stock; the risk of litigation related to the proposed transaction; the risk that the credit ratings of the combined company or its subsidiaries may be different from what the companies expect; the diversion of management time from ongoing business operations and opportunities as a result of the proposed transaction; the risk of adverse reactions or changes to business or employee relationships, including those resulting from the announcement or completion of the proposed transaction; adverse economic conditions; losses on media purchases and production costs; reductions in spending from Omnicom or IPG clients, a slowdown in payments by such clients, or a deterioration or disruption in the credit markets; risks related to each company’s ability to attract new clients and retain existing clients; changes in client advertising, marketing, and corporate communications requirements; failure to manage potential conflicts of interest between or among clients of each company; unanticipated changes related to competitive factors in the advertising, marketing, and corporate communications industries; unanticipated changes to, or any inability to hire and retain key personnel at either company; currency exchange rate fluctuations; reliance on information technology systems and risks related to cybersecurity incidents; risks and challenges presented by utilizing artificial intelligence technologies and related partnerships; changes in legislation or governmental regulations; risks associated with assumptions made in connection with critical accounting estimates and legal proceedings; risks related to international operations; risks related to environmental, social, and governance goals and initiatives; and other risks inherent in Omnicom’s and IPG’s businesses.

All such factors are difficult to predict, are beyond Omnicom’s and IPG’s control, and are subject to additional risks and uncertainties, including those detailed in Omnicom’s annual report on Form 10-K for the year ended December 31, 2023, quarterly reports on Form 10-Q, and current reports on Form 8-K that are available on its website at https://investor.omnicomgroup.com/financials/sec-filings/default.aspx and on the SEC’s website at http://www.sec.gov, and those detailed in IPG’s annual report on Form 10-K for the year ended December 31, 2023, quarterly reports on Form 10-Q and current reports on Form 8-K that are available on IPG’s website at https://investors.interpublic.com/sec-filings/financial-reports and on the SEC’s website at http://www.sec.gov.

Forward-looking statements are based on the estimates and opinions of management at the time the statements are made. Neither Omnicom nor IPG undertakes any obligation to publicly update any forward-looking statement, whether as a result of new information, future events or otherwise, except as required by law. Readers are cautioned not to place undue reliance on these forward-looking statements that speak only as of the date hereof.

NO OFFER OR SOLICITATION

This communication is not intended to be, and shall not constitute, an offer to buy or sell or the solicitation of an offer to buy or sell any securities, or a solicitation of any vote or approval, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made, except by means of a prospectus meeting the requirements of Section 10 of the U.S. Securities Act of 1933, as amended.

IMPORTANT ADDITIONAL INFORMATION WILL BE FILED WITH THE SEC

In connection with the proposed transaction, Omnicom and IPG intend to file a joint proxy statement with the SEC and Omnicom intends to file with the SEC a registration statement on Form S-4 that will include the joint proxy statement of Omnicom and IPG and that will also constitute a prospectus of Omnicom. Each of Omnicom and IPG may also file other relevant documents with the SEC regarding the proposed transaction. This document is not a substitute for the joint proxy statement/prospectus or registration statement or any other document that Omnicom or IPG may file with the SEC. The definitive joint proxy statement/prospectus (if and when available) will be mailed to stockholders of Omnicom and IPG. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE REGISTRATION STATEMENT, JOINT PROXY STATEMENT/PROSPECTUS AND ANY OTHER RELEVANT DOCUMENTS THAT MAY BE FILED WITH THE SEC, AS WELL AS ANY AMENDMENTS OR SUPPLEMENTS TO THOSE DOCUMENTS, CAREFULLY AND IN THEIR ENTIRETY IF AND WHEN THEY BECOME AVAILABLE BECAUSE THEY CONTAIN OR WILL CONTAIN IMPORTANT INFORMATION ABOUT OMNICOM, IPG AND THE PROPOSED TRANSACTION.

Investors and security holders will be able to obtain free copies of the registration statement and joint proxy statement/prospectus (if and when available) and other documents containing important information about Omnicom, IPG and the proposed transaction, once such documents are filed with the SEC through the website maintained by the SEC at http://www.sec.gov. Copies of the registration statement and joint proxy statement/prospectus (if and when available) and other documents filed with the SEC by Omnicom may be obtained free of charge on Omnicom’s website at https://investor.omnicomgroup.com/financials/sec-filings/default.aspx or, alternatively, by directing a request by mail to Omnicom’s Corporate Secretary at Omnicom Group Inc., 280 Park Avenue, New York, New York 10017. Copies of the registration statement and joint proxy statement/prospectus (if and when available) and other documents filed with the SEC by IPG may be obtained free of charge on IPG’s website at https://investors.interpublic.com/sec-filings/financial-reports or, alternatively, by directing a request by mail to IPG’s Corporate Secretary at The Interpublic Group of Companies, Inc., 909 Third Avenue, New York, NY 10022, Attention: SVP & Secretary.

PARTICIPANTS IN THE SOLICITATION

Omnicom, IPG and certain of their respective directors and executive officers may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information about the directors and executive officers of Omnicom, including a description of their direct or indirect interests, by security holdings or otherwise, is set forth in Omnicom’s annual report on Form 10-K for the year ended December 31, 2023, including under the heading “Information About Our Executive Officers,” and proxy statement for Omnicom’s 2024 Annual Meeting of Stockholders, which was filed with the SEC on March 28, 2024, including under the headings “Executive Compensation,” “Omnicom Board of Directors,” “Directors’ Compensation for Fiscal Year 2023” and “Stock Ownership Information.” To the extent holdings of Omnicom common stock by the directors and executive officers of Omnicom have changed from the amounts reflected therein, such changes have been or will be reflected on Initial Statements of Beneficial Ownership of Securities on Form 3 (“Form 3”), Statements of Changes in Beneficial Ownership on Form 4 (“Form 4”) or Annual Statements of Changes in Beneficial Ownership of Securities on Form 5 (“Form 5”), subsequently filed by Omnicom’s directors and executive officers with the SEC. Information about the directors and executive officers of IPG, including a description of their direct or indirect interests, by security holdings or otherwise, is set forth in IPG’s annual report on Form 10-K for the year ended December 31, 2023, including under the heading “Executive Officers of the Registrant,” and proxy statement for IPG’s 2024 Annual Meeting of Stockholders, which was filed with the SEC on April 12, 2024, including under the headings “Board Composition,” “Non-Management Director Compensation,” “Executive Compensation” and “Outstanding Shares and Ownership of Common Stock.” To the extent holdings of IPG common stock by the directors and executive officers of IPG have changed from the amounts reflected therein, such changes have been or will be reflected on Forms 3, Forms 4 or Forms 5, subsequently filed by IPG’s directors and executive officers with the SEC. Other information regarding the participants in the proxy solicitations and a description of their direct and indirect interests, by security holdings or otherwise, will be contained in the registration statement and joint proxy statement/prospectus and other relevant materials to be filed with the SEC regarding the proposed transaction when such materials become available. Investors and security holders should read the registration statement and joint proxy statement/prospectus carefully when it becomes available before making any voting or investment decisions. You may obtain free copies of any of the documents referenced herein from Omnicom or IPG using the sources indicated above.