Rivallo () was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He came to power in 817BC.Monarchie Nobelesse website, Bretons
He was the son of King Cunedagius and was noted as a young king who reigned with moderation. Geoffrey describes him as a "peaceful and fortunate youth, who ruled the kingdom well".Geoffrey of Monmouth: The History of the Kings of Britain : an Edition and Translation of De Gestis Britonum (Historia Regum Britanniae), Boydell & Brewer, 1 Jan 2007, p.44.
His reign was troubled by natural disasters: a rain of blood that lasted three days, a devastating plague, and a great swarm of flies. He was succeeded by his son Gurgustius.
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