Caradocus (middle Welsh: Karadawc), according to Geoffrey of Monmouth's Historia regum Britanniae, a pseudohistorical account of the kings of the Britons, was the duke of Cornwall under the reign of Octavius, who became king of Cornwall and died during the Emperor Magnus Maximus' reign.
When Octavius agreed to the idea, Caradocus sent out his son, Mauricius, to Rome as to deliver the message to Maximus.
Conan Meriadoc, the king's nephew, did not approve and nearly attacked Maximus when he landed near Southampton.
Five years after Maximus became king of Britain, he left the country to ravage the land of Gaul, and Geoffrey of Monmouth says that Maximus had left governance of his kingdom to Caradocus' brother, Dionotus, whom he calls the king of Cornwall, "who had succeeded his brother Caradoc in that kingdom."
Cornish antiquary Richard Carew has Caradocus as Carodoc Duke of Cornwall, and gives an earlier source (D. Kay) who says that he was tasked by Octavius to found the University of Cambridge in 443.