Frederick III of Simmern, the Pious, Elector Palatine of the Rhine (14 February 1515 – 16 October 1576) was a ruler from the house of Wittelsbach, specifically the cadet branch of Palatinate-Simmern-Sponheim.

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.

To get a clear understanding of the controversy Frederick spent days and nights in theological studies and was thus led more and more to the Reformed confession.

A disputation held in June 1560 between the Saxon theologians Johann Stössel and Joachim Mörlin and the Heidelbergers Pierre Boquin, Thomas Erastus, and Paul Einhorn increased Frederick's dislike for the Lutheran zealots.

The Heidelberg Catechism, prepared by a committee of theologians and ministers likely led by Ursinus, now served as the norm of doctrine and for the instruction of the youth.

Frederick, however, declared in the 14 May session of the Diet that a matter was concerned over which God alone has the rule, and if it was intended to proceed against him, he would find comfort in the promises of his Saviour.

Frederick pronounced the sentence of death on the Antitrinitarian Johann Sylvan based on the opinion signed by Olevianus, Ursinus, and Boquin, on 23 December 1572.

Frederick had carved out a territory from the Lower Palatine land dubbed "Pfalz-Lautern" for his second surviving son Johann Casimir as an enclave to enable the continued existence of the Reformed faith.