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.

The 6th-century monk Gildas mentions Constantine in chapters 28 and 29 of work De Excidio et Conquestu Britanniae. Constantine is one of five Brittonic kings whom the author rebukes and compares to Biblical beasts. Gildas calls Constantine the "tyrannical whelp of the unclean lioness of Damnonia", a reference to the books of Daniel and Revelation, and apparently also a slur directed at his mother. This Damnonia is generally identified as the kingdom of Dumnonia in present-day South West England. Scholars such as Lloyd Laing and Leslie Alcock note the possibility that Gildas may have instead intended the territory of the Damnonii, a tribe in present-day Scotland mentioned by Ptolemy in the 2nd century, but others such as Thomas D. O'Sullivan consider this unlikely.

Gildas says that despite swearing an oath against deceit and tyranny, Constantine disguised himself in an abbot's robes and attacked two "royal youths" praying before a church altar, killing them and their companions. Gildas is clear that Constantine's sins were manifold even before this, as he had committed "many adulteries" after casting off his lawfully wedded wife. Gildas encourages Constantine, whom he knows to still be alive at the time, to repent his sins lest he be damned. The murders may relate to a 6th-century cult in Brittany honoring the Saints Dredenau, two young princes killed by an ambitious uncle.

Scholars generally identify Gildas' Constantine with the figure Custennin Gorneu or Custennin Corneu (Constantine of Cornwall) who appears in the genealogies of the kings of Dumnonia. Custennin is mentioned as the father of Erbin and the grandfather of the hero Geraint in the Bonedd y Saint, the prose romance Geraint and Enid, and after emendation, the genealogies in Jesus College MS 20. Based on Custennin's placement in the genealogies, Thomas D. O'Sullivan suggests a floruit for Constantine of 520–523.

The historical Constantine of Dumnonia may have influenced later traditions, known in southwestern Britain as well as in Wales, Ireland, and Scotland, about a Saint Constantine who is usually said to have been a king who gave up his crown to become a monk. The Cornish and Welsh traditions especially may have been influenced by Gildas, in particular his adjuration for Constantine to repent; the belief may have been that the reproach eventually worked.

The two major centers for the cultus of Saint Constantine were the church in Constantine Parish and the Chapel of Saint Constantine in St Merryn Parish (now Constantine Bay), both in Cornwall. The former was established by at least the 11th century, as it is mentioned in Rhygyfarch's 11th-century Life of Saint David. At this time it may have supported a clerical community, but in later centuries it was simply a parish church. The Chapel at Constantine Bay had a holy well, and was the center of its own sub-parish.

The Annales Cambriae (Welsh Annals) and the Annals of Ulster record the conversion of a certain Constantine; these may be a reference to the Cornish saint and therefore to the historical figure. Several subsequent religious texts refer to Constantine, generally associating him with Cornwall, often specifically as its king. The Life of Saint David says that Constantine, King of Cornwall, gave up his crown and joined Saint David's monastery at Menevia. The Vitae Petroci includes an episode in which Saint Petroc protects a stag being hunted by a wealthy man named Constantine, who eventually converts and becomes a monk. Here Constantine is not said to be king, but a 12th-century text referring to this story, the Miracula, specifically names him as such, further adding that upon his conversion he gave Petroc an ivory horn that became one of the saint's chief relics. A number of other traditions attested across Britain describe saints or kings named Constantine, suggesting a confusion and conflation of various figures.