Fritz John (14 June 1910 – 10 February 1994) was a German-born mathematician specialising in partial differential equations and ill-posed problems.

Following Hitler's rise to power in 1933 "non-aryans" were being expelled from teaching posts, and John, being half Jewish, emigrated from Germany to England.

He stayed at Kentucky until 1946, apart from between 1943 and 1945, during which he did war service for the Ballistic Research Laboratory at the Aberdeen Proving Ground in Maryland.

Throughout the 1940s and 1950s he continued to work on the Radon transform, in particular its application to linear partial differential equations, convex geometry, and the mathematical theory of water waves.

On 5 May 1985, jointly with Olga Arsenievna Oleinik,[2] he was awarded the laurea honoris causa in mathematics by the Sapienza University of Rome.