Gwydion fab Dôn (Welsh pronunciation: [ˈɡwɨ̞djɔn vaːb ˈdoːn]) is a magician, hero and trickster of Welsh mythology, appearing most prominently in the Fourth Branch of the Mabinogi, which focuses largely on his relationship with his young nephew, Lleu Llaw Gyffes. He also appears prominently in the Welsh Triads, the Book of Taliesin and the Stanzas of the Graves.

The name Gwydion may be interpreted as "Born of Trees".

Gilfaethwy, nephew to the Venedotian king, Math fab Mathonwy, becomes obsessed with his uncle's virgin foot-holder, Goewin. His brother Gwydion conspires to start a war between the north and the south. To this end, Gwydion employs his magic powers to steal a number of otherworldly pigs from the Demetian king, Pryderi, who retaliates by marching on Gwynedd. During the war, while Math is distracted, the brothers return home and Gilfaethwy rapes Goewin.

Pryderi and his men march north and fight a battle between Maenor Bennardd and Maenor Coed Alun, but are forced to retreat. He is pursued to Nant Call, where more of his men are slaughtered, and then to Dol Benmaen, where he suffers a third defeat. To avoid further bloodshed, it is agreed that the outcome of the battle should be decided by single combat between Gwydion and Pryderi. The two contenders meet at a place called Y Velen Rhyd in Ardudwy, and "because of strength and valour and magic and enchantment", Gwydion triumphs and Pryderi is killed. The men of Dyfed retreat back to their own land, lamenting over the death of their lord.

When Math hears of the assault on Goewin, he turns his nephews into a series of mated pairs of animals: Gwydion becomes a stag for a year, then a sow and finally a wolf. Gilfaethwy becomes a hind, a wild boar and a she-wolf. Each year they produce an offspring which is sent to Math: Hyddwn, Hychddwn and Bleiddwn. After three years, Math releases his nephews from their punishment and begins the search for a new foot-holder. Gwydion suggests his sister Arianrhod, who is magically tested for virginity by Math. During the test, she gives birth to a "sturdy boy with thick yellow hair" whom Math names Dylan and who takes on the nature of the seas until his death at his uncle Gofannon's hands.

Ashamed, Arianrhod runs to the door, but on her way out something small drops from her, which Gwydion wraps up and places in a chest at the foot of his bed. Some time later, he hears screams from within the chest, and opens it to discover a baby boy. Some scholars have suggested that in an earlier form of the Fourth Branch, Gwydion was the father of Arianrhod's sons.

Some years later, Gwydion accompanies the boy to Caer Arianrhod, and presents him to his mother. The furious Arianrhod, shamed by this reminder of her loss of virginity, places a tynged on the boy: that only she could give him a name. Gwydion however tricks his sister by disguising himself and the boy as cobblers and luring Arianrhod into going to them in person in order to have some shoes made for her. The boy throws a stone and strikes a wren "between the tendon and the bone of its leg", causing Arianrhod to make the remark "it is with a skillful hand that the fair-haired one has hit it ". At that Gwydion reveals himself, saying Lleu Llaw Gyffes; "the fair-haired one with the skillful hand," is his name now". Furious at this trickery, Arianrhod places another tynged on Lleu: he shall receive arms from no one but Arianrhod herself. Gwydion tricks his sister once again, and she unwittingly arms Lleu herself, leading to her placing a third tynged on him: that he shall never have a human wife.

So as to counteract Arianrhod's curse, Math and Gwydion: