Dunvallo Molmutius (Welsh: Dyfual moel myd) was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He was the son of Cloten , the King of Cornwall, and he restored order after the Civil War of the Five Kings. He and his descendants were of a sub-branch of the genealogical line of Brutus, the dominant line having ended with Porrex I before the civil war.

Dunvallo was the King of Cornwall during the war created in the power vacuum left by Porrex I. He was braver and more courageous than all the other kings in the war. He defeated Pinner, the king of Loegria. In response, Rudaucus, king of Kambria, and Staterius, king of Albany, allied together and destroyed much of Dunvallo's land. The two sides met in battle and were stalemated. Dunvallo then took 600 of his men and himself and dressed themselves in the armour of the dead enemies. They led a charge deep into enemy lines where they killed the two kings. After this battle, Dunvallo destroyed the remaining defenses of the kings and pillaged their lands.

Following the defeat of the rival kings, Dunvallo created a crown like that of his predecessors and claimed the throne of Britain. He created a set of rules for the kingdom called the Molmutine Laws which lasted for many centuries. Also, robbery throughout the kingdom nearly ended for fear of Dunvallo and his laws. He reigned in peace and prosperity for forty years then died and was buried in the Temple of Concord, a tribute to his laws, which resided in Trinovantum. His death sparked another Civil War between his two sons, Belinus and Brennius.