The Dumnonii is the Latin name for a Celtic tribe which emerged in the wake of the Roman withdrawal from England during the 4th century AD.

Their territory spanned Somerset, Devon and Cornwall with further holdings in Dorset. They were possibly the most powerful of all the British Celtic tribes. It is also possible that the Dumnonii held some Breton land at some point. The name of Dumnonia in Celtic is Dunein or Dyfnaint (the latter is more accurately used just for Devon). Cornwall is called Cernyw in Welsh. They appear to have had a presence also in Ireland, as shown by the presence of a people called the Fir Domnann in the province of Connacht

Originally their capital would have been at Exeter (known to the Romans as "Isca" but to the Britons as "Caeresk"), and there is evidence that the Celts continued to live in Exeter until (at least) the tenth century as equals with the newer Saxon arrivals. A part of Exeter retained the title 'Little Britain' until the eighteenth century.

Dumnonia was believed by some to be effectively a collection of sub-kingdoms. As the eastern boundary of Brythonic Dumnonia receded to the west, the tribe's history becomes indistinguishable from that of the Kingdom of Cornwall.