John Casimir, Count Palatine of Simmern 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.

Frederick III of Simmern, the Pious, Elector Palatine of the Rhine was a ruler from the house of Wittelsbach, branch Palatinate-Simmern-Sponheim. He was a son of John II of Simmern and inherited the Palatinate from the childless Elector Otto-Henry, Elector Palatine (Ottheinrich) in 1559. He was a devout convert to Calvinism, and made the Reformed confession the official religion of his domain by overseeing the composition and promulgation of the Heidelberg Catechism. His support of Calvinism gave the German Reformed movement a foothold within the Holy Roman Empire.