Paul Sophus Epstein (Warsaw, Vistula Land, Russian Empire, March 20, 1883 – Pasadena, California, United States, February 8, 1966) was a Russian-American mathematical physicist. He was known for his contributions to the development of quantum mechanics, part of a group that included Lorentz, Einstein, Minkowski, Thomson, Rutherford, Sommerfeld, Röntgen, von Laue, Bohr, de Broglie, Ehrenfest and Schwarzschild.