Cador (Latin: Cadorius) was a legendary Duke of Cornwall, known chiefly through Geoffrey of Monmouth's pseudo-historical History of the Kings of Britain and related Welsh sources. As the son of Gorlois and Ygraine, he would be a maternal half_brother of King Arthur. Furthermore, Arthur's future queen Guinevere was for a time his ward. Cador's son Constantine was given the kingship of Britain by Arthur as the latter lay ailing on the field of Camlann.

In the Brut Tysilio, a mediaeval Welsh adaption of the Historia Regum Britanniae, the translator adds the information that Cador was son of Gorlois, one would guess by Igraine. The same appears in Richard Hardyng's Chronicle where Cador is called Arthur's brother "of his mother's syde." Different views appear in Layamon's Brut where Cador appears first as a leader who takes charge of Uther's host when they are attacked by Gorlois while Uther is secretly lying beside Ygraine in Tintagel. In the English Alliterative Morte Arthure Cador is continually called Arthur's "cousin".