In magician appearing prominently in the Fourth Branch of the Mabinogi and the ancient poem ''Cad Goddeu''. He is the brother of Gilfaethwy and Arianrhod, and the nephew of Math fab Mathonwy. In the ''Mabinogion'' he is called the son of the goddess Dôn, making it likely he is an euhemerized god or demi-god. The name Gwydion may mean 'To Speak Poetry'.

==Mythological exploits==
In the Fourth Branch of the Mabinogi, Gwydion helps his brother Math's foot-holder. To this end he steals turns his nephews into a series of mated pairs of animals; Gwydion becomes a sow and finally a hind deer, a 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.

In the search for a new foot-holder, who must be a virgin, Math tests Gwydion's sister Arianrhod. The test reveals that Arianrhod is not a virgin, however, when she immediately gives birth to two children after stepping over Math's wand: Dylan Ail Don and a unformed blob.

Dylan is a sea-creature who immediately moved into the ocean, but on the other child Arianrhod places three ''Irish ''geis'') upon him: the child will never have a name unless she herself names him, he cannot carry weapons unless she arms him (neither of these things does she intend to do), and he cannot marry any human woman. In effect she denies her child three major aspects of humanity, but Gwydion puts his nephew in a box and raises him. When the boy is old enough Gwydion takes him incognito to see Arianrhod, who declares he is a "bright one with a sure hand" or in some versions "fair-haired skillful hand" when she sees him drop a wren with a single stone. Gwydion reveals the child is her son and that she has unknowingly supplied him with a name; from then on he goes by Lleu Llaw Gyffes, "bright one with a sure hand". Arianrhod is similarly tricked into supplying her son with weapons. The third curse proves harder to overcome, so Gwydion and Math use magic to create a wife for Lleu out of flowers, named Blodeuwedd (flower face). Blodeuwedd proves unfaithful and with her lover, Goronwy, attempts to slay Lleu. Lleu does not die but transforms into a wounded eagle, and Gwydion tracks him with the help of a pig and finds him perched on an oak. He calls Lleu down from the tree by singing an englyn known as englyn Gwydion, returns Lleu back to his human form and with the help of Math heals him. They return to Lleu's estate where Gwydion turns Blodeuwedd into an owl, and Lleu himself kills Goronwy.

Gwydion also appears in the 6th century poem ''Cad Goddeu'' (The Battle of the Trees), found in the ''Book of Taliesin''. There he wins a battle against Bendigeidfran by animating an army of trees and guessing Bendigeidfran's name.

==Gwydion in modern fiction==
Taran when the Assistant Pig-Keeper chases after Horned King by shouting his real name, leads the attempt to gain the Black Cauldron and helps Taran, Fflewddur Fflam, Gurgi and Prince Rhun rescue Princess Eilonwy from Achren.  It is Gwydion who leads an assault on Annuvin by the sea shortly after he becomes High King.  In the books, Prince Gwydion is an expert tracker, forester and warrior.  As a member of the Royal House of Don, he often wears a pendant depicting a simple golden disk meant to represent the sun.  (The character is not included in the Disney animated film based on the novels.)

Keltiad'' series - Irish, Welsh and Scottish legends translated to an interstellar, "Star Wars" style context - has a character named Gwydion Prince of Don as its co-protagonist. Like Alexander, Kennealy-Morrison bases her character on the mythological Gwydion, but humanizes him through her own creative process. Lover and First Lord of War to the Queen of Keltia, Aeron Aoibhell, Gwydion ultimately becomes Aeron's husband and King of Keltia, while having numerous adventures based on episodes from the various branches of the Mabinogion (Spiral Castle, the magic fleet, etc.). He is a gifted bard, sorcerer and warrior, close to (and descended from) the magical Sidhefolk of Keltia, and is portrayed as Aeron's true and loving partner and her equal in most things. He takes a major part in the epic battle that ends "The Throne of Scone", the chronological last of the books published.

He also appears in alternate history series ''A Land Fit for Heroes'', Robert Carter's "The Language of Stones" series, has a short appearance in ''American Gods'' by Neil Gaiman and is the ancestor to the main character in Jenny Nimmo's ''Snow Spider Trilogy''. In ''The Mists of Avalon'', Gwydion is the birth name of both King Arthur and Mordred. The name Gwydion also appears in the Sierra game King's Quest III, where a Prince Alexander of Daventry has been kidnapped by an evil wizard named Manannan who renames him Gwydion.

Gwydion is also one of the main protagonists in the books of the Welsh author Jenny Sullivan.

Dans la Quatrième branche du Mabinogi'' et dans un poème faussement attribué à Taliesin, le ''Kat Godeu'' (le « Combat des « arbrisseaux »).

== Parenté ==

Gwydion est le fils de la déesse suprême Dôn, il est le frère de Amaethon, Gilfaethwy, Gofannon Arianrhod. Selon Christian-J. Guyonvarc'h et Françoise Le Roux,<ref>''Les Druides'', page 397,  Ouest-France Université, coll. « De mémoire d’homme : l’histoire », Rennes, 1986, .</ref> ils correspondent aux principaux dieux de la mythologie celtique irlandaise, les Tuatha Dé Danann, Gwydion étant l’équivalent du Dagda et Arianrhod celui de Brigit.

Il est le neveu de Math ap Mathonwy.

== Exploits mythologiques ==

Dans la Quatrième Branche du Mabinogi, Gwydion aide son frère Gilfaethwy à violer Goewin, la « porte-pieds » de Math<ref>Math ne peut vivre que les pieds dans le giron d'une vierge, excepté pendant les périodes de guerre.</ref>. Pour ce faire, il vole les cochons de Pryderi de Dyfed, forçant Math à partir en guerre et donc quitter sa position. Gwydion et Gilfaethwy rentrent alors discrètement à la cour de Math où Gilfaethwy couche de force avec Goewin. Quant Math, qui est aussi un magicien, apprend cela il transforme les coupables en couples de différents animaux. Gwydion devient un cerf pendant une année, puis une truie l’année suivante et finalement un loup ; Gilfaethwy, quant à lui,  est transformé en biche, en sanglier puis en louve. De ces métamorphoses naissent un faon (Hyddwn), un marcassin (Hychtwn) et un louveteau (Bleiddwn). Au bout de trois ans, Math relève ses neveux de leur châtiment.