King Arthur (Welsh: Brenin Arthur, Cornish: Arthur Gernow, Breton: Roue Arzhur) is a legendary king of Britain, and a central figure in the medieval literary tradition known as the Matter of Britain. In Welsh sources, Arthur is portrayed as a leader of the post-Roman Britons in battles against Anglo-Saxon invaders of Britain in the late 5th and early 6th centuries. He first appears in two early medieval historical sources, the Annales Cambriae and the Historia Brittonum, but these date to 300 years after he is supposed to have lived, and most historians who study the period do not consider him a historical figure. His name also occurs in early Welsh poetic sources such as Y Gododdin. The character developed through Welsh mythology, appearing either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh otherworld Annwn.

The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). Geoffrey depicted Arthur as a king of Britain who defeated the Saxons and established a vast empire. Many elements and incidents that are now an integral part of the Arthurian story appear in Geoffrey's Historia, including Arthur's father Uther Pendragon, the magician Merlin, Arthur's wife Guinevere, the sword Excalibur, Arthur's conception at Tintagel, his final battle against Mordred at Camlann, and final rest in Avalon. The 12th-century French writer Chrétien de Troyes, who added Lancelot and the Holy Grail to the story, began the genre of Arthurian romance that became a significant strand of medieval literature. In these French stories, the narrative focus often shifts from King Arthur himself to other characters, such as various Knights of the Round Table. The themes, events and characters of the Arthurian legend vary widely from text to text, and there is no one canonical version. Arthurian literature thrived during the Middle Ages but waned in the centuries that followed, until it experienced a major resurgence in the 19th century. In the 21st century, the legend continues to have prominence, not only in literature but also in adaptations for theatre, film, television, comics and other media.