In Welsh mythology, Gwydion was a son of the goddess Don. He helped Gilfaethwy rape Goewin, Math ap Mathonwy's foot-holder. He accomplished this by stealing Pryderi of Dyfed's pigs, thus sending Math away to fight a war. Gwydion and Gilfaethwy were supposed to be with him, but they snuck back. Gwydion forced Goewin to stay with Gilfaethwy and he raped her. She told Math, and he turned them into animals. Gwydion was, for one year each, a stag, sow and wolf. Gilfaethwy was, for one year each, a hind, boar and she-wolf. Each year, they had a child together and Math turned the three children into people.

During Cad Goddeu (the Battle of the Trees), Gwydion turned trees into warriors, thus winning the war begun by his brother, Amaethon.

Gwydion raised his nephew, who was born a blob. The blob was placed in a chest by Gwydion. Arianrhod, the blob's mother, created three geases: only she could give him a name; only she could give him weapons; he would have no human wife. Arianrhod denied him the three aspects of masculinity. Gwydion raised him anyway, even without a name. Later Arianrhod saw him killing a wren with a single stone. She said that he was a bright lion with a sure hand and he took the name Llew Llaw Gyffes ("bright lion with a sure hand"). Gwydion then tricked her into arming him. Llew created his own woman out of flowers, Blodeuwedd. After Llew Llaw Gyffes was killed by Blodeuwedd, Gwydion resurrected Llew and turned her into an owl.

In Lloyd Alexander's Chronicles of Prydain, a series of fantasy novels inspired by Welsh myths, there is a character named Gwydion, who is based somewhat on the Gwydion of myth, but is markedly different in terms of moral character.

In Marion Zimmer Bradley's The Mists of Avalon, Gwydion was the birth-name of both King Arthur and Mordred.