Dumnonia is the Latinized name for the Brythonic kingdom in sub-Roman Britain between the late 4th and late 8th centuries, located in the farther parts of the south-west peninsula of Great Britain.

It is centered in the area later known as Devon, but also includes parts of Somerset and possibly Dorset, its eastern boundary changing over time.Some historians interpret it as including Cornubia or Cornwall, although the kingdom of Cornwall is based on a sub-tribe, the Cornovii, and appears to have been at least semi-independent at times, certainly retaining its independence after parts of Dumnonia come under Anglo-Saxon control between the 7th and 10th centuries.