Riothamus (also spelled Riotimus, Rigothamus, Rigotamos), was a military leader, active circa 470, called "King of the Brittones" by Jordanes, who states in The Origin and Deeds of the Goths:

If the name is a Latinization of "highest leader", some scholars have suggested, it may be a title, and not a personal name. It has been argued whether Jordanes' "Brittones" refers to the Bretons of Brittany, a Briton colony in Armorica in northwestern Gaul, or of the Britons of Great Britain itself. Jordanes states that they "came ... by way of the Ocean", which has been taken to mean that he was a leader in Great Britain or even the leader of the British people on both sides of the English Channel.

He took part in the Roman campaign against Euric, king of the Visigoths. Euric defeated his attack, and Riothamus vanishes from history among the BurgundiansProponents of an Arthurian connection would have him "retreating towards a small town in Burgundy called Avallon", though no contemporary source makes any connection with Avallon in Burgundy..

A letter has survived that was written to Riothamus from Sidonius Apollinaris, bishop of Clermont, who requested his judgment for "an obscure and humble person" who has had his slaves enticed away by a group of armed Bretons (see Ref.).

Riothamus has been identified as a candidate for the historical King Arthur by some recent scholars (notably Geoffrey AsheThe Discovery of King Arthur, Guild Publishing, London, 1985 and Leon Fleuriot). In any case, Riothamus' activities in Gaul may be the seed from whence grew the tradition (first recorded by Geoffrey of Monmouth in his Historia Regum Britanniae) that Arthur crossed the English Channel from Britain and attacked Rome.

His downfall is theorized to come from the betrayal of his prefect, Arvandus. After Riothamus' departure to Gaul with his Briton army, Arvandus gathers his own army in Riothamus' absence and rebels, in alliance with the Goths Ashe, Geoffrey. A Certain Very Ancient Book: Traces of an Arthurian Source in Geoffrey of Monmouth's History. Speculum. 1981.

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