Cador (Latin: Cadorius) was a legendary Duke of Cornwall, known chiefly through Geoffrey of Monmouth's pseudo-historical Historia Regum Britanniae and previous manuscript sources such as the Life of Carantoc. Early sources present Cador as a relative of King Arthur, though the details of their kinship are usually left unspecified.
Cado was the historical son of a Dumnonian king named Gerren whom he succeeded as monarch. Traditionally he was a good friend of Arthur; they even ruled together in the Vita Sanctus Carantoci (Life of St. Carantoc). He also seemed to share a good relationship with King Caradoc of Gwent. Possibly he gave his name to four hillforts, all named Cadbury which may be "Cado's fort", one each near to Clevedon, Congresbury and Sparkford in Somerset and one by the Exe in Devon north of Crediton). Cadsonbury hillfort lies just outside Callington, also known as Celliwig in Cornwall.
In Geoffrey's History and elsewhere, Arthur's future queen Guinevere was raised as Cador's ward. Cador is also said to be of Roman stock. His son Constantine was given the kingship of Britain by Arthur as the latter lay ailing on the field of Camlann. To the Brut Tysilio the translator adds the information that Cador was son of Gorlois, presumably by Igraine, which would make him Arthur's maternal half-brother. The same account appears in Richard Hardyng's Chronicle where Cador is called Arthur's brother "of his mother's syde." In Layamon's Brut Cador appears as a leader who takes charge of Uther's host when they are attacked by Gorlois while Uther is secretly lying beside Igraine in Tintagel. Most works, such as the English Alliterative Morte Arthure and Sir Thomas Malory's Le Morte d’Arthur, however, call Cador Arthur's "cousin".
In The Dream of Rhonabwy, a medieval romance associated with the Mabinogion Cador is "Cadwr Earl of Cornwall, the man whose task it is to arm the king on the day of battle and conflict" – i.e. at the Battle of Badon Hill, which the writer situates close to the upper River Severn.