Dionotus was a legendary king of the Britons during the campaigns in Gaul led by Emperor Magnus Maximus, according to Geoffrey of Monmouth. The curious thing about this king is that the Welsh chronicles, which parallel most of Monmouth's book, does not mention this king by name. He succeeded his brother, Caradocus as ruler of the Britons and Cornwall.

Nothing is said of Dionotus until he became king under Maximus. He is first mentioned when Conan Meriadoc sends a request to Britain for Briton women to help populate Brittany, of which he was the king. Dionotus, being extremely noble and powerful, accepted the request and sent out seventy-two thousand women to Gaul to help populate Brittany. The ships, however, became lost at sea and most of the women died or were captured by barbarians.

A small group of these women defied kings Wanius and Melga of the Picts and the Huns, who attempted to have intercourse with them. The women were slaughtered for their defiance and the kings invaded Britain in Albany. Britain, due to the war led by Maximus and the tragedy at sea, was empty of all able-bodied men and women. This allowed the two kings room to destroy much of the countryside before any attempt at resisting them could be made.

Maximus finally sent a man named Gracianus Municeps to stop the attack by the kings. He was sent in with two legions and killed many thousands of warriors before the kings fled off the island to Ireland. Maximus died in Rome soon after and Dionotus became the official king of the Britons. Unfortunately, before he could begin his reign, Gracianus took hold of the crown and made himself king over Dionotus. No mention of Dionotus's fate is made.

Geoffery may have created this character from the historical figure of Marcus, a short lived usurper emperor in Britain whose limited historical exploits are seemingly mirrored by 'Dionotus'.