Dionotus was a legendary king of Cornwall in Geoffrey of Monmouth's Historia regum Britanniae, an account of the rulers of Britain based on ancient Welsh sources and disputed by many historians. Dionotus succeeding his brother Caradocus, and was 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 Geoffrey of Monmouth's book, do not mention this king by name. However, Geoffrey uses Latin versions of Welsh names so he could be referring to Dynod, duke of Cornwall, or Anwn Dynod, Maximus's own son. The latter would probably be Conan Meriadoc another son of Caradoc of Cornwall.