Geraint, King of Dumnonia, ruled in the early eighth century. During his reign, Wessex is believed to have conquered the lands that became the county of Devon in a series of battles that culminated in a victory of Wessex and Sussex forces under Ine in AD 710.

A letter survives addressed to him from Aldhelm on the Easter Problem. It is clear that in the later seventh century the British in Cornwall and Devon still observed Easter on the dates the Celtic church had calculated, at variance with Catholic practice.

He was the last recorded king of Dumnonia.