A post-Roman British kingdom that covered Devon, Cornwall and other areas of the south-west of England, which survived the Roman invasion. Its most westerly potion being known as Cornubia to the Romans.

In the sixth century, it was apparently ruled by a King Constantine, who becomes Arthurís successor on Geoffrey of Monmouth. Its name undoubtedly comes from the Dumnonii, a British tribe who were more than likely related to the Irish Fir Dhomhnann.