Geraint (known in Latin as Gerontius) (died 710) was a King of Dumnonia who ruled in the early 8th century. During his reign, it is believed that Dumnonia came repeatedly into conflict with neighbouring Anglo-Saxon Wessex. Geraint was the last recorded king of a unified Dumnonia, and was called King of the Welsh by the Anglo-Saxon Chronicle. Subsequent kings of Dumnonia (for example Donyarth and possibly Huwal) reigned over a reducing area that eventually encompassed only the present day Cornwall.
A long and rather acrimonious letter survives addressed to him from Aldhelm, Bishop of Sherborne on the Easter Problem and the shape of the tonsure. It is clear from this letter that in the later 7th century the Britons in Cornwall and Devon still observed Easter on the dates that the British church had calculated, at variance with Roman practice. Geraint ultimately agreed with Aldhelm to comply with Roman practice on these points. According to John of Worcester, Geraint was killed after a series of battles that culminated in a victory of the West Saxons under Ine of Wessex in 710. It was probably around this time that Devon was conquered by the West Saxons. After Geraint's death, however, Ine was unable to establish his authority over neighbouring Cornwall; in 722, according to the Annales Cambriae, the Cornish won the Battle of Hehil, probably against Wessex.
Derek Bryce, following other scholars, suggests Geraint of Dumnonia be identified as the warrior eulogized in the poem Battle of Llongborth, traditionally ascribed to Llywarch Hen. Other scholars associate the slain man with Geraint ab Erbin, a king said to have lived during the 5th century but of dubious historicity. Bryce identifies Llongborth with the 710 battle between Geraint and Ine, and suggests Langport in Somerset as the location of the battle.