Achille Grassi (1456–1523) was an Italian Roman Catholic bishop and cardinal.
Achille Grassi was born in Bologna on February 16, 1456, the son of Baldassarre Grassi, a Bolognese patrician, and Orsina Bocchi.
He attended the University of Bologna, becoming a doctor of both laws in 1478. He was also a canon of the cathedral chapter of Bologna Cathedral.
He then traveled to Rome, joining his uncle Antonio, who was an auditor of the Roman Rota. When his uncle died in 1491, Pope Innocent VIII appointed him an auditor of the Roman Rota. In 1503, he became a chaplain of Pope Julius II. The pope also named him rector of San Clemente, San Giovanni in Persiceto. He then became a Referendary in the Roman Curia.
He was consecrated as a bishop in Rome by Pope Julius II on February 13, 1506. The next day, he was elected Bishop of Città di Castello. He occupied this see until 1516, when he resigned in favor of Cardinal Giulio de' Medici, the future Pope Clement VII.