Benedetto Castelli (born Antonio Castelli, 1578–1643) was an Italian mathematician and astronomer. Born in Brescia, he studied at the University of Padua and later became an abbot at the Benedictine monastery in Monte Cassino. He was a long-time friend and supporter of his teacher, Galileo Galilei. He assisted Galileo's study of sunspots and participated in the examination of the theories of Nicolaus Copernicus.