James (Jim) Lepowsky (born 5 July 1944 in New York City) is a professor of mathematics at Rutgers University, New Jersey. Previously he taught at Yale University. He received his Ph.D from M.I.T. in 1970 where his advisor was Bertram Kostant. Lepowsky graduated from Stuyvesant High School in 1961, 16 years after Kostant. His current research is in the areas of infinite-dimensional lie algebras and vertex algebras. He has written several books on vertex algebras and related topics. In 1988, in a joint work with Igor Frenkel and Arne Meurman, he constructed the monster vertex algebra (also known as Moonshine module). His students include Yi-Zhi Huang and Arne Meurman.
In 2012 he became a fellow of the American Mathematical Society.