Dionotus, in Geoffrey of Monmouth's Historia regum Britanniae, whose account of the rulers of Britain is based on ancient Welsh sources disputed by many historians, was the "legendary" king of Cornwall, succeeding his brother Caradocus, and regent of Britain during the campaigns in Gaul of Emperor Magnus Maximus. The curious thing about this king is that the Welsh chronicles, which parallel most of Monmouth's book, do not mention this king by name although Monmouth uses Latin versions of Welsh names so he could be referring to Dynod, duke of Cornwall or Anwn Dynod, Maximus' own son. The latter would probably be Conan Meriadoc another son of Caradoc of Cornwall.
Nothing is said of Dionotus until he became king under Maximus. He is first mentioned when Conan Meriadoc, king of Brittany sends a request to Britain for Briton women to help populate his country. Dionotus, being extremely noble and powerful, accepted the request and sent seventy-two thousand women to Gaul. The ships, however, became lost at sea and most of the women died or were captured by barbarians.
No further mention is made of Dionotus by Monmouth, but 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 from 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 while no mention is made of Dionotus or his fate, Gracianus took hold of the crown.