Benedetto Castelli, born Antonio Castelli (Brescia, 1578 – Rome, 1643), took the name "Benedetto" upon entering the Benedictine Order in 1595.

Benedetto Castelli 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.

Castelli was interested in mathematics and hydraulics. He was appointed as a mathematician to the University of Pisa and later at the University of Rome. He published Mensuration of Running Water, and important work on fluids in motion.

One of his students was Evangelista Torricelli, the inventor of the barometer.