Riothamus (also spelled Riutimus or Riotimus) is a Romano-British military leader, who is active circa 470.

He fights against the Goths in alliance with the declining Roman Empire.

He is called "King of the Britons" by the 6th-century historian Jordanes, but the extent of his realm is unclear.

Riothamus is a Latinization of the Brythonic personal name *Rigotamos, meaning 'king-most', 'supreme king' or 'highest king'.

Though it is still a matter of debate, several scholars consider his life to have been one of the possible sources for the King Arthur legend.