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

Emil Joseph Diemer was born in 1908 in the German town Radolfzell, in Baden.

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.

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.