Emil Joseph (Josef) Diemer (15 May 1908, in Radolfzell – 10 October 1990, in Fussbach/Gengenbach) was a German chess master.

In 1953, he was expelled from the German Chess Federation, whose officials he had accused, in a press campaign, of "homosexuality and corruption of innocent youth".

The clinic's director, believing that chess was excessively stressful for Diemer, banned him from playing the game.

In 1971, however, this ban was rescinded, and Diemer's membership in the German Chess Federation was also reinstated.

Still lacking financial independence, however, he continued to reside in Gengenbach as a semi-residential patient of the hospital until the end of his life.