Constantine (, , fl. 520–523) was a 6th-century king of Dumnonia in sub-Roman Britain, who was remembered in later British tradition as a legendary King of Britain. The only contemporary information about him comes from Gildas, who castigated him for various sins, including the murder of two "royal youths" inside a church. The historical Constantine is also known from the genealogies of the Dumnonian kings, and possibly inspired the tradition of Saint Constantine, a king-turned-monk venerated in Southwest Britain and elsewhere.