Zacharias Ursinus (18 July 1534 – 6 May 1583) was a sixteenth-century German Reformed theologian, born Zacharias Baer in Breslau (now Wrocław, Poland). He became the leading theologian of the Reformed Protestant movement of the Palatinate, serving both at the University of Heidelberg and the College of Wisdom (Collegium Sapientiae). He is best known as the principal author and interpreter of the Heidelberg Catechism.