Cadwaladr ap Cadwallon or Cadwaladr Fendigaid (Welsh pronunciation: [kadˈwaladr vɛnˈdɪgai̯d], c. 633 – 682) was the king of Gwynedd from after 655 to 682. Little is known of Cadwaladr's reign, but he later became a mythical redeemer figure in medieval Welsh literature following his depiction in the De gestis Britonum by Geoffrey of Monmouth. In Geoffrey's narrative, Cadwaladr was the last native Briton to be King of Britain, and renounced his throne in 689 to go on pilgrimage to Rome in response to a prophecy that his sacrifice of personal power would bring about a future victory of the Britons over the Anglo-Saxons. However, Geoffrey's account of Cadwaladr's sanctity and visit to Rome is the result of a conflation with historical events in the life of Cædwalla of Wessex.

For later Welsh writers, the myth provided hope in a period where the native order was increasingly finding itself encroached upon by and subject to English authority and customs. However, because of Geoffrey's popularity in England, the legend was also used by both the Yorkist and Lancastrian factions during the Wars of the Roses to claim that their candidate would fulfil the prophecy by restoring the authentic lineage of Cadwaladr to the throne of England. From the sixteenth century onwards, the Welsh Dragon has sometimes been conflated with Cadwaladr's legend and referred to as "Red Dragon of Cadwalader" because of the importance of both Cadwaladr and the dragon in the ideology of Henry Tudor's supporters which helped to justify his claim to the throne.

There are no contemporary records of Cadwaladr or his reign, and those which do survive are confused and contradictory. Peter Bartrum suggested that he may have been born about 633 AD, shortly before his father's death at the Battle of Heavenfield. The earliest Welsh genealogies contained in the ninth-century manuscript Harley 3859 record him simply as Catguala[tr] map Catgollaun and trace his ancestry back to Cunedda Wledig.

The earliest narration of Cadwaladr's life is the Historia Brittonum, written in 829 or 830 AD, but its depiction of Cadwaladr is not internally consistent. In the section of the Historia Brittonum known as the "Northern History" because of its synchronisation of events with the reigns of kings of Northumbria, it is said that Cadwaladr reigned after his father Cadwallon ap Cadfan, who died in battle against Oswald at Heavenfield. However, the following section states that the king of Gwynedd in 655 was Cadafael ap Cynfeddw, who was allied with Penda of Mercia but abandoned him on the eve of the Battle of the Winwæd and thus earned the nickname Cadomedd 'Battle Dodger'. The Historia Brittonum states that Cadwaladr died of plague in the reign of Oswiu. However, Owsiu died in 670, and an entry in Welsh Annals records Cadwaladr's death as having occurred as a result of a plague in 682. This discrepancy arose because the author of the Historia Brittonum wrongly associated the plague in the Welsh Annals with the 664 plague in Bede's Ecclesiastical History. The author's confusion may have occurred because the Welsh Annals do not contain any absolute dates but instead numbers its annals in groups of ten. Cadwaladr’s reign is the last of a Welsh ruler in the Historia Brittonum, and this terminal position within such a widely disseminated text may have significantly influenced both his legacy and his portrayal in subsequent medieval literature. It is uncertain if Cadwaladr was succeeded by his son Idwal Iwrch.

A king called Cadualadrus is described as the final ruler of the Britons in Geoffrey of Monmouth’s De gestis Britonum, a widely circulated fictional narrative of British history written between 1137 and 1139. Geoffrey drew on Welsh sources to fashion his narrative. However, his figure of Cadualadrus is an amalgam of the historical Cadwaladr and the Anglo-Saxon king Cædwalla, a late seventh-century ruler of the West Saxons.

In De gestis Britonum, Cadualadrus is said to have been the son of Caduallo, based on the Cadwallon ap Cadfan, and a half-sister of Penda by a different mother but the same father. After a plague depopulates Britain, Cadualadrus leaves Britain for Brittany, and discerns that it is God's will that the sovereignty of the Britain be taken from the British and given over to others. This plague completely depopulated Britain south of Scotland except for Cornwall and Wales, with the Anglo-Saxons settling the empty lands in what would become England. After the plague had passed, Cadualadrus prepared a fleet to recover Britain for the native Britons, but an angel appeared to Cadualadrus and told him that God would not allow the native Britons to rule over Britain until "the time came which Merlin had foretold to Arthur". Furthermore, Cadualadrus was ordered by the angel to visit Pope Sergius in Rome, where after doing penance, his sainthood was assured. The angel further promised that sovereignty of the island would be returned to the Britons when Cadualadrus' body was returned to Britain from Rome sometime in the future. Cadualadrus' Breton host Alanus consulted written prophecies and urged Cadualadrus to fulfil the angel's words and go to Rome, where after taking monastic vows Cadualadrus died on 20 April 689.

However, Geoffrey obtained the story of Cadualadrus' pilgrimage and even his date of death from Bede's account of Cædwalla of the West Saxons, who did historically die whilst on pilgrimage in Rome on 20 April 689. As the year 689 would have been within the same group of ten years as the 682 entry in the Welsh Annals, Geoffrey would have experienced the same aforementioned difficulty as did the author of the Historia Brittonum in understanding the date of Cadwaladr's death. If the merging of the two men does not go back to before De gestis Britonum, then Geoffrey seems, at minimum, to have elevated Cadwaladr’s holiness, if he did not reimagine a completely secular figure. This conflation has been argued to predate Geoffrey's time. However, it was Cadwaladr's father Cadwallon who bore a name equivalent to Cædwalla, not Cadwaladr himself, and so a combination of his life and and that of Cædwalla of the West Saxons is not likely to be a blunder.

In 1150, Geoffrey of Monmouth completed the Vita Merlini, a poem narrating how Merlin goes mad after a battle and thereafter dwells in woods and prophesies the future. Geoffrey's Merlin foretells the restoration of Britain to the native Britons, mentioning Cadwaladr by name: