Constantine (/ˈkɒnstəntiːn/, Welsh: Cystennin, 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.
In the 12th century, Geoffrey of Monmouth included Constantine in his pseudohistorical chronicle Historia Regum Britanniae, adding details to Gildas' account and making Constantine the successor to King Arthur as King of Britain. Under Geoffrey's influence, Constantine appeared as Arthur's heir in later chronicles. Less commonly, he also appeared in that role in medieval Arthurian romances and prose works, and in some modern versions of the legend.