UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A INFORMATION Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by a Party other than the Registrant [ ] Filed by the Registrant ☒ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☒ Definitive Proxy Statement ☐ Definitive Additional Materials ☐ Soliciting Material Pursuant to §240.14a-12 THE RUBICON PROJECT, INC. (Name of Registrant as Specified In Its Charter) (Name of Person(s) Filing Proxy Statement, if other than the Registrant) Payment of Filing Fee (Check the appropriate box): ☒ No fee required ☐ Fee computed on table below per Exchange Act Rules 14a-6(i)(1) and 0-11 1) Title of each class of securities to which transaction applies: 2) Aggregate number of securities to which transaction applies: 3) Per unit price or other underlying value of transaction computed pursuant to Exchange Act Rule 0-11 (set forth the amount on which the filing fee is calculated and state how it was determined): 4) Proposed maximum aggregate value of transaction: 5) Total fee paid: ☐ Fee paid previously with preliminary materials. ☐ Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing. 1) Amount Previously Paid: 2) Form, Schedule or Registration Statement No.: 3) Filing Party: 4) Date Filed: The Rubicon Project, Inc. 12181 Bluff Creek Drive, 4th Floor Los Angeles, California 90094 NOTICE OF ANNUAL MEETING OF STOCKHOLDERS TO BE HELD ON JULY 8, 2020 The annual meeting of stockholders of The Rubicon Project, Inc. (the “company”) will be held on Wednesday, July 8, 2020, at 12:00 noon Pacific time, to consider and act upon the matters described below. In light of the coronavirus (COVID-19) pandemic and the protocols that federal, state and local governments are currently imposing, and out of an abundance of caution and appreciation for our stockholders, this year’s annual meeting will be a virtual meeting via live webcast on the Internet. You will be able to attend the annual meeting, vote and submit your questions during the meeting by visiting https://web.lumiagm.com/293659257 and entering the control number included on the Notice of Internet Availability or the proxy card or voting instruction form (if you received a printed copy of the proxy materials) that you receive. You will not be able to attend the annual meeting in person. 1. Election of three Class III directors to serve until the company’s 2023 annual meeting of stockholders and until their respective successors are duly elected and qualified. 2. Ratification of the selection of Deloitte & Touche LLP as the company’s independent registered public accounting firm for the current fiscal year. 3. Approval, on an advisory basis, of the compensation of the company’s named executive officers. 4. Approval, on an advisory basis, of the frequency of future advisory votes on the compensation of the company’s named executive officers. 5. Transaction of such other business as may properly come before the meeting or any postponement or adjournment thereof. Stockholders of record at the close of business on May 14, 2020 will be entitled to notice of and to vote at the meeting or any postponement or adjournment thereof. WHETHER OR NOT YOU EXPECT TO ATTEND THE ANNUAL MEETING, IN ORDER TO ENSURE REPRESENTATION OF YOUR SHARES, PLEASE VOTE AS PROMPTLY AS POSSIBLE. YOU ARE URGED TO SUBMIT YOUR PROXY OR VOTING INSTRUCTIONS ELECTRONICALLY OR BY TELEPHONE BY FOLLOWING THE INSTRUCTIONS ON YOUR NOTICE OF INTERNET AVAILABILITY OF PROXY MATERIALS OR, IF YOU RECEIVED A PRINTED COPY OF THE PROXY MATERIALS, ON YOUR PROXY CARD OR VOTING INSTRUCTION FORM. IF YOU REQUESTED A PRINTED COPY OF YOUR PROXY MATERIALS, YOU MAY ALSO VOTE BY MAIL BY SIGNING, DATING, AND RETURNING YOUR PROXY CARD OR VOTING INSTRUCTION FORM IN THE PRE-PAID ENVELOPE PROVIDED. VOTING NOW VIA PROXY WILL NOT LIMIT YOUR RIGHT TO CHANGE YOUR VOTE OR TO ATTEND THE ANNUAL MEETING. By Order of the Board of Directors, Aaron Saltz Corporate Secretary Los Angeles, California May 27, 2020 SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS This proxy statement contains forward-looking statements, including statements based upon or relating to our expectations, assumptions, estimates, and projections. In some cases, you can identify forward-looking statements by terms such as “may,” “might,” “will,” “objective,” “intend,” “should,” “could,” “can,” “would,” “expect,” “believe,” “design,” “anticipate,” “estimate,” “predict,” “potential,” “plan” or the negative of these terms, and similar expressions. These statements are not guarantees of future performance; they reflect our current views with respect to future events and are based on assumptions and estimates and subject to known and unknown risks, uncertainties and other factors that may cause our actual results, performance or achievements to be materially different from expectations or results projected or implied by forward-looking statements. These risks include, but are not limited to: • the severity, magnitude, and duration of the novel coronavirus pandemic, including impacts of the pandemic and of responses to the pandemic by governments, business and individuals on our operations, personnel, buyers, sellers, and on the global economy and the advertising marketplace; • our ability to successfully integrate the Telaria business and realize the anticipated benefits of the merger; • our ability to grow and to manage our growth effectively; • our ability to develop innovative new technologies and remain a market leader; • our ability to attract and retain buyers and sellers of digital advertising inventory, or publishers, and increase our business with them; • our vulnerability to loss of, or reduction in spending by, buyers; • our reliance on large sources of advertising demand, including demand side platforms (“DSPs”) that may have or develop high- risk credit profiles or fail to pay invoices when due, including as a result of lower ad spending generally and/or general liquidity constraints experienced by buyers resulting from the novel coronavirus pandemic; • our ability to maintain and grow a supply of advertising inventory from sellers and to fill the increased inventory; • the effect on the advertising market and our business from difficult economic conditions or uncertainty; • the freedom of buyers and sellers to direct their spending and inventory to competing sources of inventory and demand; • our ability to cause buyers and sellers to use our solution to purchase and sell higher value advertising and to expand the use of our solution by buyers and sellers utilizing evolving digital media platforms, including CTV; • our reliance on large aggregators of advertising inventory, and the concentration of CTV among a small number of large publishers that enjoy significant negotiating leverage; • our ability to introduce new offerings and bring them to market in a timely manner, and otherwise adapt in response to client demands and industry trends, including shifts in linear TV to CTV, digital advertising shift from desktop to mobile channels and other platforms and from display to video formats and the introduction and market acceptance of Demand Manager; • uncertainty of our estimates and expectations associated with new offerings, including CTV, header bidding, private marketplace, mobile, video, Demand Manager, and traffic shaping; • the possibility of lower take rates and the need to grow through increasing the volume and/or value of transactions on our platform and increasing our fill rate; • our vulnerability to the depletion of our cash resources as a result of the adverse impacts of the novel coronavirus pandemic, or as we incur additional investments in technology required to support the increased volume of transactions on our exchange and to develop new offerings; • our ability to support our growth objectives with reduced resources from our cost reduction initiatives; • our ability to raise additional capital if needed and/or renew our working capital line of credit; • our limited operating history and history of losses; • our ability to continue to expand into new geographic markets and grow our market share in existing markets; • our ability to adapt effectively to shifts in digital advertising; • increased prevalence of ad-blocking or cookie-blocking technologies and the slow adoption of common identifiers; • the slowing growth rate of desktop display advertising; • the growing percentage of online and mobile advertising spending captured by owned and operated sites (such as Facebook, Google, and Amazon); • industry growth rates for ad-supported CTV and the shift in video consumption from linear TV to digital mediums such as CTV and over-the-top ("OTT"); • the adoption of programmatic advertising by CTV publishers; • the effects, including loss of market share, of increased competition in our market and increasing concentration of advertising spending, including mobile spending, in a small number of very large competitors; • the effects of consolidation in the ad tech industry; • acts of competitors and other third parties that can adversely affect our business; • our ability to differentiate our offerings and compete effectively in a market