King Arthur and King Cornwall in the context of The Marriage of Sir Gawain

The Green Knight (Welsh: Marchog Gwyrdd, Cornish: Marghek Gwyrdh, Breton: Marc'heg Gwer) is a heroic character of the Matter of Britain, originating in the 14th-century poem Sir Gawain and the Green Knight and the related medieval work The Greene Knight. His true name is revealed to be Bertilak de Hautdesert (spelled in some translations as "Bercilak" or "Bernlak") in Sir Gawain, while The Greene Knight names him "Bredbeddle".The Green Knight later features as one of Arthur's greatest champions in the fragmentary ballad King Arthur and King Cornwall, again with the name "Bredbeddle".

In Sir Gawain and the Green Knight, Bertilak is transformed into the Green Knight by Morgan le Fay, a traditional adversary of King Arthur, to test his court. However, in The Greene Knight, he is transformed by a different woman for the same purpose. In both stories, he sends his wife to seduce Gawain as a further test. The King Arthur and King Cornwall ballad portrays him as an exorcist and one of the most powerful knights of Arthur's court. His wider role in Arthurian literature includes being a judge and tester of knights, and as such, the other characters consider him as friendly but terrifying and somewhat mysterious.