Riothamus (also spelled Riotimus, Rigothamus, Rigotamos), was a military leader and considered "King of the Brittones" (c.470). Because the name means "highest leader", some scholars have suggested it may be a title, and not a personal name. The name Riothamus is also very similar to "Rio Tâmisa" in Portuguese and Spanish which means Thames river. It has usually been assumed that the "Brittones" or "Brittani" refers to the Bretons or the people of Brittany, a British colony in Armorica in northern Gaul, but the reference by Jordanes in his The origin and deeds of the Goths which states that they "came ... by way of the Ocean" could mean that he was a leader in mainland Britain or even the leader of the British people on both sides of the English Channel.

He took part in the Western Roman Emperor Anthemius' campaign against Euric, king of the Visigoths. Euric defeated his attack, and Riothamus vanishes from history while retreating towards a small town in Burgundy called Avallon. A letter to Riothamus from Sidonius Apollinaris, who requested support to act against troublemaking Bretons, has survived.

Riothamus has been identified as a source for the historical King Arthur by some recent scholars (notably Geoffrey Ashe 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.