Thomas Erastus (September 7, 1524 – December 31, 1583) was a Swiss physician and theologian best known for a posthumously published work in which he argued that the sins of Christians should be punished by the state, and not by the church withholding the sacraments. A generalization of this idea, that the state is supreme in church matters, is known somewhat misleadingly as Erastianism.
Erastus was born of poor parents, probably at Baden (later part of the Canton of Aargau) in Switzerland. He translated his original surname, Lüber, to "Erastus" in humanist style.
In 1540 he was studying arts and theology at Basel. After surviving the plague in 1544, he moved to Bologna as student of philosophy and medicine. In 1553 he became physician to the count of Henneberg, Saxe-Meiningen, and in 1558 held the same post with the elector-palatine, Otto Henry, Elector Palatine, being at the same time professor of medicine at Heidelberg. His patron's successor, Frederick III, made him (1559) a privy councillor and member of the church consistory.