Dylan ail Don (Middle Welsh pronunciation [ˈdəlan ˈail ˈdɔn]) is a character in the Welsh mythic Mabinogion tales, particularly in the fourth tale, "Math fab Mathonwy". The story of Dylan reflects ancient Celtic myths that were handed down orally for some generations before being written down during the early Christian period by clerics. The story as it has been preserved therefore exhibits elements and archetypes characteristic of both Celtic pagan and Christian mythologies. His name translates as "Dylan the Second Wave."

In some interpretations of legend, Dylan represents darkness while his twin brother Lleu Llaw Gyffes represents light. But the more common interpretation is that Dylan is a Welsh sea-god. Dylan was killed by his uncle, and it has been said that "The clamour of the waves dashing upon the beach is the expression of their longing to avenge their son."

In Wales, Dylan is one of the most popular traditional Welsh names for boys.

In Math fab Mathonwy it is told that Dylan's great uncle Math would die if he did not keep his feet in the lap of a virgin when not at war. Math's original foot-holder, Goewin, is raped by Gilfaethwy who is punished severely when Math returns, turning Gilfaethwy and Gwydion into a series of mated pairs of animals. Math then marries Goewin to alleviate her shame, but must find a new virgin to hold his feet. Gwydion suggests his sister, Arianrhod. Math magically tests Arianrhod to confirm that she is a virgin, at which point she gives birth to twin sons.

One of the sons, Lleu Llaw Gyffes, is borne away by Gwydion as a "lump of flesh" and concealed in a chest until maturity, but the other, a sturdy blonde boy, was immediately forsaken by his mother yet was acknowledged by his great uncle Math and given the name Dylan. As soon as Dylan comes in contact with his baptismal waters, he plunges into the sea and takes on characteristics of a sea creature, moving through the seawater as perfectly as any fish:

'So they had the boy baptised, and as they baptised him he plunged into the sea. And immediately when he was in the sea, he took its nature, and swam as well as the best fish that was therein. And for that reason was he called Dylan, the son of the Wave'

Dylan is accidentally killed by his uncle Gofannon in the end.

'And the blow whereby he came to his death, was struck by his uncle Gofannon. The third fatal blow was it called'.