Ernst Rexer (2 April 1902 – 14 May 1983) was a German nuclear physicist. He worked on the German nuclear energy program during World War II. After the war, he was sent to Laboratory V, in Obninsk, to work on the Soviet atomic bomb project. In 1956, he was sent to East Germany, where he was a professor and director of the Institute for the Application of Radioactive Isotopes at the Technische Hochschule Dresden.