One of Arthur’s warriors in Culhwch and Olwen. He was the son of Greidawl. Gwythyr helped Culhwch by collecting the bottles of Linseed Oil requested by the Chief Giant Ysbaddaden. The bottles were given to Gwythyr by a hill of ants after he saved the ants from a fire. Gwythyr also participated in the search for the Black Hag in the Valley of Distress.

Gwythyr loved the lady Creiddylad, daughter of Lludd Silver Hand, and he took her from her father’s house. Before he could consummate their relationship, Gwynn son of Nudd rescued her from Gwythyr. Gwythyr raised an army to oppose Gwynn but was defeated. Meanwhile, Arthur intervened and returned Creiddylad to her father’s house, and it was arranged that each leader should fight until Doomsday on each May Day, to end the senseless bloodshed. The eventual winner gaining the hand of Creiddylad. This arrangement was later said to have been made by one other than King Arthur himself

A Welsh Triad names Gwythyr as the father of one of Arthur’s three wives named Gwenhwyfar.