The Dumnonii or Dumnones were a British tribe who inhabited Dumnonia, the area now known as Cornwall and Devon (and some areas of present-day Dorset and Somerset) in the further parts of the South West peninsula of Britain, from at least the Iron Age up to the early Saxon period. They were bordered to the east by the Durotriges tribe.

William Camden, in his 1607 edition of Britannia, describes Cornwall and Devon as being two parts of the same 'country' which:

was in ancient time inhabited by those Britains whom Solinus called Dunmonii, Ptolomee Damnonii, or (as we find in some other copies) more truly Danmonii. ... . But... the Country of this nation is at this day divided into two parts, known by later names of Cornwall and Denshire [Devonshire] ... The near or hithermore region of the Danmonians that I spake of is now commonly called Denshire, [or] by the Cornish-Britains 'Dewnan', and by the Welsh Britains 'Duffneint' [sic], that is, 'low valleys', for that the people dwell for the most part beneath in Vales; by the English Saxons [it is known as] 'Deven-schire', whereof grew the Latin name 'Devonia', and by that contraction which the vulgar people useth, 'Denshire'.

Camden had learnt some Welsh during the course of his studies and it would appear that he is the origin of the interpretation of Dumnonii as "deep valley dwellers" from his understanding of the Welsh of his time. The modern Welsh term is Dyfnaint. John Rhŷs later theorized that the tribal name was derived from the name of a goddess, Domnu, probably meaning "the goddess of the deep". The proto-Celtic root *dubno- or *dumno- meaning "the deep" or "the earth" (or alternatively meaning "dark" or "gloomy") appears in personal names such as Dumnorix and Dubnovellaunus. Another group with a similar name but with no known links were the Fir Domnann of Connacht.

The Roman name of the town of Exeter, Isca Dumnoniorum ("Isca of the Dumnonii"), contains the root *iska- "water" for "Water of the Dumnonii". The Latin name suggests that the city was already an oppidum, or walled town, on the banks on the River Exe before the foundation of the Roman city, in about AD 50. The Dumnonii gave their name to the English county of Devon, and their name is represented in Britain's two extant Brythonic languages as Dewnens in Cornish and Dyfnaint in Welsh. Amédée Thierry (Histoire des Gaulois, 1828), one of the inventors of the historic race of Gauls, could confidently equate them with the Cornish ("les Cornouailles").

Victorian historians often referred to the tribe as the Damnonii, which is also the name of another people from lowland Scotland, although there are no known links between the two populations.

The people of Dumnonia spoke a Southwestern Brythonic dialect of Celtic similar to the forerunner of more recent Cornish and Breton. Irish immigrants, the Déisi, are evidenced by the Ogham-inscribed stones they have left behind, confirmed and supplemented by toponymical studies. The stones are sometimes inscribed in Latin, sometimes in both scripts. Tristram Risdon suggested the continuance of a Brythonic dialect in the South Hams, Devon, as late as the 14th century, in addition to its use in Cornwall.

Ptolemy's 2nd century Geography places the Dumnonii to the west of the Durotriges. The name purocoronavium that appears in the Ravenna Cosmography implies the existence of a sub-tribe called the Cornavii or Cornovii, perhaps the ancestors of the Cornish people.