Gregory Breit (Russian: Григорий Альфредович Брейт-Шнайдер, Grigory Alfredovich Breit-Shneider; July 14, 1899, Mykolaiv, Kherson Governorate – September 13, 1981, Salem, Oregon) was a Russian-born Jewish American physicist and professor at New York University (1929–1934), University of Wisconsin–Madison (1934–1947), Yale University (1947–1968), and University at Buffalo (1968–1973). In 1921, he was Paul Ehrenfest's assistant in Leiden University.