Frederick III of Simmern, Elector Palatine of the Rhine (February 14, 1515 – October 26, 1576) is a ruler from the house of Wittelsbach, branch Palatinate-Simmern-Sponheim.
He is a son of John II of Simmern and inherits the Palatinate from the childless Elector Otto-Henry, Elector Palatine (Ottheinrich) in 1559.
He is a devout convert to Calvinism, and makes Calvinism the official religion of his domain.
Under his supervision the Heidelberg Catechism is constructed.
His support of Calvinism gives the religious sect a foothold within the Holy Roman Empire.