John Casimir, Count Palatine of Simmern (German: Johann Casimir von Pfalz-Simmern) (7 March 1543 – ) was a German prince and a younger son of Frederick III, Elector Palatine. A firm Calvinist, he was a leader of mercenary troops in the religious wars of the time, including the Dutch Revolt. From 1583–1592 he acted as regent for his nephew, Elector Palatine Frederick IV.