Thomas Erastus (original surname Lüber, Lieber, or Liebler;[2] 7 September 1524 – 31 December 1583) was a Swiss physician and Calvinist theologian.

In 1553 he became physician to the count of Henneberg, Saxe-Meiningen, and in 1558 held the same post with the elector-palatine, Otto Heinrich, being at the same time professor of medicine and pharmacology at the University of Heidelberg until 1580; Anselmus Boetius de Boodt was one of his students.

[4] In theology he followed Huldrych Zwingli, and at the sacramentarian conferences of Heidelberg (1560) and Maulbronn (1564) he advocated by voice and pen the Zwinglian doctrine of the Lord's Supper, replying in 1565 to the counter-arguments of the Lutheran Johann Marbach, of Strasbourg.

He ineffectually resisted the efforts of the Calvinists, led by Caspar Olevian, to introduce the Presbyterian polity and discipline, which were established at Heidelberg in 1570, on the Geneva model.

Its immediate occasion was the disputation at Heidelberg in 1568 for the doctorate of theology by George Withers, an English Puritan (subsequently Archdeacon of Colchester), silenced in 1565 at Bury St Edmunds by Archbishop Parker.

An appendix of letters to Erastus by Heinrich Bullinger and Rudolf Gwalther, showed that the Theses, written in 1568, had been circulated in manuscript form.