Brian Russell Leiter (/ˈlaɪtər/; born 1963) is an American philosopher and legal scholar who is Karl N. Llewellyn Professor of Jurisprudence in the Law School and affiliated faculty in the Department of Philosophy at the University of Chicago, as well as founder and Director of Chicago's Center for Law, Philosophy & Human Values. A review in Notre Dame Philosophical Reviews described Leiter as "one of the most influential legal philosophers of our time", while a review in The Journal of Nietzsche Studies described Leiter's book Nietzsche on Morality (2002) as "arguably the most important book on Nietzsche's philosophy in the past twenty years."

Leiter taught from 1995 to 2008 at the University of Texas School of Law, where he was the founder and director of the Law and Philosophy Program. He joined the University of Chicago faculty in 2008. His scholarly writings have been primarily in legal philosophy and Continental philosophy, especially Nietzsche and Marx. He has also been a visiting professor at universities in the United States and Europe, including Yale University and Oxford University. He is founding editor of a book series entitled Routledge Philosophers, and (with Leslie Green) of Oxford Studies in Philosophy of Law.

Leiter was also the founder and for 25 years the editor of the Philosophical Gourmet Report ("PGR"), an influential but controversial ranking of philosophy PhD programs in the English-speaking world. After repeated protests, one in 2002 and one in 2014, Leiter retired and turned over editorship of the PGR to Berit Brogaard, a philosopher at the University of Miami, and Christopher Pynes, a philosopher at Western Illinois University.

Born to a Jewish family in Manhattan, Leiter earned his Bachelor of Arts in philosophy from Princeton University (1984), and his J.D. (1987) and Ph.D. (in philosophy; 1995) from the University of Michigan, where his dissertation was supervised by Peter Railton.

Leiter taught for two years at the University of San Diego School of Law, and was a visiting assistant professor of philosophy at the University of California, San Diego, before joining the faculty at the University of Texas School of Law in 1995, where he taught until 2008. At Texas, Leiter was the founder and Director of the Law and Philosophy Program. In 2008, Leiter moved to the University of Chicago, where he became Karl N. Llewellyn Professor of Jurisprudence at the University of Chicago Law School, and founder of Chicago's Center for Law, Philosophy & Human Values.

Leiter has been a visiting professor of law or philosophy at Yale Law School, University College London, University of Chicago Law School, University of Paris X-Nanterre, University of California, San Diego, and Oxford University. He edited the journal Legal Theory from 2000 to 2008, and is editor of the Routledge Philosophers, a series of introductions to major philosophers, and (with Leslie Green) of Oxford Studies in the Philosophy of Law.

Cato Institute senior fellow Walter K. Olson described Leiter as "left-leaning" in his book Schools for Misrule: Legal Academia and an Overlawyered America (Encounter Books, 2011), and Leiter himself has professed sympathy for Marx, stating in an interview, "On two central issues, Marx was far more right than any of his critics: first, that the long-term tendency of capitalist societies is towards immiseration of the majority (the post-WWII illusion of upward mobility for the 'middle classes' will soon be revealed for the anomaly it was); and second, that capitalist societies produce moral and political ideologies that serve to justify the dominance of the capitalist class."

Leiter's scholarly writings have been in two main areas—legal philosophy and Continental philosophy—although he has also written about metaethics, religious liberty, and other topics. Philosophical naturalism has been a major theme in many of these contexts.