Mark J. Penn (born January 15, 1954) is an American businessman, pollster, political strategist, and author.

Penn is chairman and chief executive officer of Stagwell, a marketing group. He was formerly chief strategy officer of Microsoft Corporation and chief executive officer of Burson-Marsteller. Penn is the author of the books Microtrends (2007) and Microtrends Squared (2018).

Together with Douglas Schoen, he was co-founder of the polling firm PSB Research, whose clients included President Bill Clinton, British prime minister Tony Blair, and Bill Gates. Penn was a chief strategist and pollster in the Hillary Clinton 2008 presidential campaign. Penn later became a defender of Donald Trump, opposing his impeachment, reportedly consulting on his 2020 presidential campaign (which Penn denies), and alleging a "deep state" conspiracy against him.

Penn was born in New York City and raised in Riverdale. His father, a Lithuanian immigrant, was a kosher poultry-plant owner, who died when Penn was 10 years old. He was raised by his mother Blanche, who worked as a schoolteacher. Both of his brothers credit Penn with keeping the family together after their father's death. Penn graduated from Horace Mann School in Riverdale in 1972. He conducted his first poll, which determined that the Horace Mann faculty was more liberal than was the country at large on the issue of civil rights, when he was 13, as well as polling classmates about sex and drugs.

Penn entered Harvard University in 1972. According to his brother, Penn was initially rejected, but he was able to get Harvard's dean to reverse the decision after taking the train to Boston and arguing his case. At Harvard, Penn majored in political science and, as a city editor of the Harvard Crimson, wrote and reported 90 articles. His work for the paper included reporting and analysis on buying a stereo, the Cambridge City Council elections of 1975, the Harvard admission process, and the controversy over the proposed construction in Cambridge of the John F. Kennedy Library. With Doug Schoen, who he attended Horace Mann School with, Penn co-founded Penn & Schoen – now the global market research firm Penn, Schoen & Berland Associates – in their dorm room. He graduated from Harvard in 1976.

While Penn was a first-year law student at Columbia University in 1976, he and his business partner Douglas Schoen became the pollsters for congressman Ed Koch's second run for mayor of New York City. In 1977, with the campaign against Mario Cuomo for the Democratic nomination in full swing, Penn sought a way to conduct polls more quickly than the mainframe and punched card system he and Schoen were making use of at Columbia University. He purchased a self-assembled "microcomputer" kit and created a program that could compile polls in a fraction of the time than had been done before. By creating this "overnight poll" system, Penn allowed the campaign to conduct polls to determine messages and evaluate tactics on a daily basis, a tactical advantage that contributed to Koch's eventual victory over Cuomo.

Penn also played a significant role in Koch's campaign during the 1985 New York City mayoral election, for which he and Schoen developed direct mailings, set up phone banks, organized volunteers and canvassers, and coordinated fundraising. That year, Koch won both the Democratic primary and the general election, defeating New York City Council President Carol Bellamy.

In 1978, Penn conducted polling for the presidential campaign of Luis Herrera Campins in Venezuela. Because Venezuela did not at that time have universal phone coverage, Penn partnered with Venezuelan polling firms to go door-to-door to collect interviews. He also helped the campaign develop the slogan "Ya Basta," or "Enough," critical of the incumbent party's spending policies. Herrera carried the election by about 3%.