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.
Galileo referred to his former student, Bendetto Castelli, during his sessions with Sigismondo Gioioso in 1633, recounting a letter Castelli sent Galileo in 1610 which sought confirmation of the movements of Venus. Galileo cited the letter as evidence of the wider acceptance of the Copernican model of the solar system.