Owain Danwyn (fl. 440) was a king of Rhos in Gwynedd, Wales, in the mid-5th century. He was the son of Einion Yrth and the father of Cynlas Goch, probably the Cuneglasus excoriated by Gildas. Very little is known of his life. Graham Phillips and Martin Keatman proposed a theory that he was the historical figure behind the legend of King Arthur.
Owain Danwyn (Old Welsh Dantguin, also spelled Ddanwyn, Ddantgwyn, etc., meaning "White Tooth"), is known from several medieval genealogies of the kings of Rhos. Most of these concur that he was the son of Einion Yrth and grandson of Cunedda, founder of the Gwynedd dynasty in North Wales. His brother was Cadwallon Lawhir ap Einion, known from the Gwynedd pedigrees. Owain was the father of Cynlas Goch, who is identified with the prince named Cuneglasus who Gildas castigated for his various sins in De Excidio et Conquestu Britanniae. According to the Bonedd y Saint, a genealogy of British saints, Owain was also the father of the saints Einion Frenin, Seiriol and Meirion, and in some versions, Hawystl Gloff.
According to Gildas, Maelgwn Gwynedd, the son of Owain's brother Cadwallon, took the throne of Gwynedd by murdering an uncle. Peter Bartrum suggests this may have been Owain, though he notes that Gildas' term avunculus typically refers to a maternal uncle.
Writers Graham Phillips and Martin Keatman proposed that Owain Danwyn was the historical basis for the legendary King Arthur. This theory is based partly on their interpretation of the British power structure in the 5th century, and interpretations of names and epithets associated with Owain's family. According to Phillips and Keatman, "Arthur" was Owain's honorific title, meaning "Bear", and his capital was Viroconium in Shropshire, England. Other scholars have criticized their evidence and conclusions.
- Bartrum, Peter (1993). "Owain Danwyn ab Einion Yrth". A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000 (PDF). National Library of Wales. p. 594. ISBN 0907158730.
- Bartrum, Peter (1993). "Cadwallon Lawhir ap Einion Yrth". A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000 (PDF). National Library of Wales. p. 94. ISBN 0907158730.
- Bartrum, Peter (1993). "Maelgwn Gwynedd". A Welsh Classical Dictionary: People in History and Legend up to about A.D. 1000 (PDF). National Library of Wales. p. 500. ISBN 0907158730.
- Wood, Charles T. (Fall 1995). "Reviewed Works: From Scythia to Camelot: A Radical Reassessment of the Legends of King Arthur, the Knights of the Round Table, and the Holy Grail by C. Scott Littleton and Linda A. Malcor, and: King Arthur: The True Story by Graham Phillips and Martin Keatman". Arthuriana. 5 (3). doi:10.1353/art.1995.0033. JSTOR 27869131.
- Castleden, Rodney (2003). King Arthur: The Truth Behind the Legend. Routledge. p. 124. ISBN 1134373775.