Owain Ddantgwyn was a prince of North Wales, probably a King of Rhos in the late 5th century.
Owain appears in various ancient Welsh genealogies as the son of Einion Yrth and the father of Cynlas Goch. One of these is given the title, 'Pedigree of (the Kings of) Rhos. According to the Bonedd y Saint, he was also the father of SS. Einion Frenin, king of Llyn, Seiriol, and Meirion. Other than these genealogies, no documentary evidence exists concerning his life.
Some modern authors have conjectured that Owain could be the origin of a "real" King Arthur. Although it has not gained wide acceptance among Arthurian scholars, this hypothesis has been proposed by Graham Phillips and Martin Keatman in their book, King Arthur: The True Story (1992). They suggest that "Arthur" was a byname (nickname) and identify its recipient as Owain from a passage in De Excidio Britanniae. Its contemporary author, Gildas, refers (in Latin) to Owain’s son, Cynlas, literally as "guider of the chariot which is the receptacle of the bear". "Bear" in Brythonic is "Arth", so Phillips and Keatman take this to infer that "the Arthur" was Cynlas’ predecessor, known from the genealogies to be Owain. They go on to claim that Owain ruled in Powys. The Owain as Arthur hypothesis draws on the relationships between Owain and his nephew Maelgwn Hir and compares it to the relationship between the Arthur and Mordred of legend. Maelgwn, the "dragon of the isle" is admonished by Gildas for murdering his uncle (Owain) and seizing his throne, events similar to those of legend whereby Mordred murders his father.
While discussing a possible connection between this Owain of Rhôs and the legend of Arthur of Britain it may also be worthy to note the similarity between Owain's epithet Ddantgwyn, meaning "white tooth", and the name of a legendary sword Dyrnwyn, meaning "white hilt".
Dyrnwyn, the sword, is associated with Rhydderch Hael (died c.614), an important king who ruled the northern Kingdom of Strathclyde after Owain Ddantgwyn's death. Rhydderch Hael was also a major protagonist in the war fought against Owain's great nephew, Rhun Hir ap Maelgwn, who had succeeded Maelgwn Hir in c.547 as the king of Gwynedd, by Strathclyde and her northern allies. In the course of this war Rhun of Gwynedd would perish in battle in Strathclyde c.586. The sword Dyrnwyn was one of the so-called Thirteen Treasures of the Island of Britain and apparently held magical properties, similar to those associated with Arthur's legendary sword, Excalibur.
- Phillips, Graham; Keatman, Martin (1992). King Arthur. The True Story. London: Arrow. ISBN 0-09-929681-0.
- Works of Gildas and Nennius, Giles, J.A. 1841, Ch.33 Gildas admonishes Maglocune