Duke of Cornwall here refers to the legendary dukes of Cornwall in Celtic Britain as established by such pseudo-historical authors as Nennius, Gildas, and above all Geoffrey of Monmouth. The list is extremely patchy, and it must not be assumed that every succession was unbroken. Indeed, Geoffroi repeatedly introduces Dukes of Cornwall only to promote them to the Kingship of the Britons and thus put an end to their line as (merely) dukes. As adjuncts or supporting roles to the kings of the Britons, the dukes of Cornwall are considered part of the vast Matter of Britain.

The list is more often thought of as a conglomeration of various Celtic rulers, Celtic warlords, and mythical heroes. If the lists of kings of Britain are mythological and apocryphal (though extraordinarily influential in mediaeval historiography), then the list of dukes must be considered still more a genealogical and historical myth with no solid basis to most historians.

Even within Geoffrey, the title of these rulers fluctuates between "duke" (dux Cornubiae) and "king" (rex Cornubiae).

For a list of Kings of Cornwall from the fifth century to the Saxon conquest, see King of Cornwall.

For a list of Dukes of Cornwall from 1337 to the present, see Duke of Cornwall.