Christopher Clavius, born Christoph Clau, (March 25, 1538 – February 12, 1612) was a German mathematician and astronomer who was the main architect of the modern Gregorian calendar. In his last years he was probably the most respected astronomer in Europe.

Clavius joined the Jesuit order in 1555 and later moved to Rome, where he taught at the order's Collegium Romanum for the rest of his life. In 1579 he was assigned to compute the basis for a reformed calendar that would stop the slow process in which the Church's holidays were drifting relative to the seasons of the year. This calendar was adopted in 1582 in Catholic countries by order of Pope Gregory XIII and is now used worldwide.

His commentary on Euclid's geometry was one of the most influential of the time.

As an astronomer Clavius held strictly to the geocentric model of the solar system, in which all the heavens rotate about the Earth. Though he opposed the heliocentric model of Copernicus, he recognized problems with the orthodox model. He was treated with great respect by Galileo, who visited him in 1611 and discussed the new observations being made with the telescope; Clavius had by that time accepted the new discoveries as genuine, though he retained doubts about the reality of the mountains on the Moon. In light of this fact, it is very ironic that a large crater on the moon is named for him.