The Dumnonii or Dumnones were a British tribe who inhabited Dumnonia, the area now known as Cornwall and Devon 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.

Dumnonia 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, with its eastern boundary changing over time as the gradual westward expansion of the neighbouring Anglo-Saxon kingdom of Wessex encroached on its territory. 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 and shares a linguistic relationship with the Breton region of Domnonée.