Karl Stein (1 January 1913 in Hamm, Westphalia – 19 October 2000) was a German mathematician.

Karl Stein received his doctorate with his dissertation on the topic Zur Theorie der Funktionen mehrerer komplexer Veränderlichen; Die Regularitätshüllen niederdimensionaler Mannigfaltigkeiten at the University of Münster under the supervision of Heinrich Behnke in 1937.

He was assigned to manage the OKW/Chi IV, Subsection a, which was a unit responsible for security of own processes, cipher devices testing, and invention of new cipher devices.

He managed a staff of 11[1] In 1955, he became professor at the Ludwig Maximilian University of Munich and emeritated in 1981.

[2] Stein's doctoral students included Michael Schneider [de], Otto Forster, Ivo Schneider, Gunther Schmidt and Martin Schottenloher.