Barry Ross Posen (born July 13, 1952) is Ford International Professor of Political Science at MIT and the director of MIT's Security Studies Program. An expert in the field of security studies, he currently serves on the editorial boards of the journals International Security and Security Studies and is a member of the Council on Foreign Relations and served as a study group member for the Hart-Rudman Commission. Posen is a structural realist.

Posen received his B.A. from Occidental College in 1974 and his M.A. and PhD, under Kenneth Waltz, from the University of California, Berkeley in 1976 and 1981 respectively. Posen worked as a consultant for the RAND Corporation and an analyst for the Department of Defense and Center for Strategic and International Studies before becoming an assistant professor of political science at Princeton University in 1984. In 1987, he joined MIT as associate professor of political science and has taught at MIT since that time. He has also served as a consultant to the Woodrow Wilson Center, The Christian Science Monitor and the MacArthur Foundation.

Posen is the author of three books.^[1] Posen has also published a number of highly influential journal articles, including "The Security Dilemma and Ethnic Conflict" and "Command of the Commons: The Military Foundations of U.S. Hegemony." In addition to his scholarly work, Posen has frequently appeared in the media and published general interest articles in publications including The New York Times, The Boston Globe, and The American Interest.

Barry Posen is not closely related to the American economist Adam Posen, although their last name has the same origin from the city of Poznań in Poland.