Boccasini entered the Order of Preachers in his native Treviso. He studied at Venice and Milan before becoming a teacher in Venice and in other Dominican houses. He served two terms as Provincial Prior of Lombardy, before being elected Master of the Order in 1296. Two years later he was made cardinal. He was appointed Bishop of Ostia, and served as papal legate first to Hungary, and then to France. He was with Pope Boniface VIII when Boniface was attacked by French forces at Anagni.