The Kingdom of Kernyw existed during the Dark Ages in Britain's southwestern peninsula. Its Latin name is Dumnonia. It was known to the English of neighbouring Wessex as the kingdom of the West Welsh, later as Cornwall. Its kings include King Mark (Marcus Cunomorus) (early 6th century), King Constantine (later known as Saint Constantine after his conversion to Christianity in 586), King Geraint (who has killed at the Battle of Llongborth (Langport) in 710), King Dumgarth (d. 878), King Hoel (c920), and King Cynan (c935).

Its territory in the 5th century and the first half of the 6th century appears to have been the modern counties of Cornwall, Devon and Somerset, and the Brythonic colony on the Continent later known as Brittany. It may possibly have included parts of Dorset, Wiltshire and Gloucestershire until the Battle of Dyrham in 577. It lost the part of Somerset east and north of the River Parrett to Wessex in 658 and the rest of Somerset in 710. The modern eastern border of Cornwall was set in 927 by Athelstan.

See also

• Legendary Dukes of Cornwall for the pseudohistoric rulers of Cornwall mentioned by Geoffrey of Monmouth