Dumnonia (Ancient Greek: Δαμνόνιον, romanized: Damnónion, in the adjective form) is the Latinised name for a Brythonic kingdom that existed in Sub-Roman Britain between the late 4th and late 8th centuries CE in the more westerly parts of present-day South West England. It was centred in the area of modern Devon, but also included modern Cornwall and part of Somerset; its eastern boundary was moved westward over time with the gradual expansion of the neighbouring Anglo-Saxon kingdom of Wessex. The spelling Damnonia is sometimes encountered, but that spelling is also used for the land of the Damnonii, later part of the Kingdom of Strathclyde, in present-day southern Scotland. The form Domnonia also occurs. The name of the kingdom shares a linguistic relationship with the Breton region of Domnonée (Breton: Domnonea).

The kingdom is named after the Dumnonii, a British Celtic tribe living in the south-west at the time of the Roman invasion of Britain, according to Ptolemy's Geography. Variants of the name Dumnonia include Domnonia and Damnonia, the latter being used by Gildas in the 6th century as a pun on "damnation" to deprecate the area's contemporary ruler Constantine. The name etymologically originates from Proto-Celtic *dubno- '(adjective) deep; (noun) world'. Groups with similar names existed in Scotland (Damnonii) and Ireland (Fir Domnann). Later, the area became known to the English of neighbouring Wessex as the kingdom of West Wales, and its inhabitants were also known to them as Defnas (i.e. men of Dumnonia). In Welsh, and similarly in the Southwestern Brythonic languages, it was Dyfneint and this is the form which survives today in the name of the county of Devon (Modern Welsh: Dyfnaint, Cornish: Dewnans, Breton: Devnent).

There is evidence, based on an entry in the Ravenna Cosmography, that there may have been a sub-tribe in the western part of the territory known as the Cornovii from whose name the first element of the present-day name of Cornwall is probably derived.

Following a period of emigration from south-western Britain to north-western Gaul (Armorica) in the 5th and 6th centuries, a sister kingdom (Domnonée in modern French), was established on the north-facing Atlantic coast of the continent in the region that was to become known as Brittany. Historian Barbara Yorke has speculated that the Dumnonii may have seen the end of the Roman empire as an opportunity to establish control in new areas. Before the arrival of the Romans, the Dumnonii seem to have inhabited the south-west peninsula of Britain as far east as the River Parrett in Somerset and the River Axe in Dorset, judging by the coin distributions of the Dobunni and Durotriges. In the Roman period there was a provincial boundary between the area governed from Exeter and those governed from Dorchester and Ilchester.^{[fact or opinion?]} Julius Caesar's Commentarii de Bello Gallico, Book III notes the close trading and military relationship between the continental Veneti of Armorica and the south-western insular British.

In the post Roman period the kingdom of Dumnonia covered Cornwall, Devon and parts of west Somerset. It had close cultural and religious links with Brittany, Wales and Ireland.

In Ptolemy's Geography, originally written in ancient greek but only preserved in medieval greek manuscripts, he records 4 towns belonging to the Δουμνόνιοι (Doumnónioi):

The cultural connections of the pre-Roman Dumnonii, as expressed in their ceramics, are thought to have been with the peninsula of Armorica across the Channel, and with Wales and Ireland, rather than with the south-east of Britain. The people of Dumnonia would have spoken a Brythonic dialect, the ancestor of modern Cornish and Breton. Irish immigrants, the Déisi, are evidenced by the inscribed stones they have left behind—sometimes written in Ogham, sometimes in Latin, sometimes in both, confirmed and supplemented by place-name studies.

Apart from fishing and agriculture, the main economic resource of the Dumnonii was tin mining, the tin having been exported since ancient times from the port of Ictis (St Michael's Mount or Mount Batten). Tin working continued throughout Roman occupation and appears to have reached a peak during the 3rd century CE. The area maintained trade links with Gaul and the Mediterranean after the Roman withdrawal, and it is likely that tin played an important part in this trade. Post-Roman imported pottery has been excavated from many sites across the region. An apparent surge in late-5th-century Mediterranean imports is thought to be related to the trade in metals from Cornwall and Wales to the Byzantine Empire.