Manawydan fab Llŷr is a figure of Welsh mythology, the son of Llŷr and the brother of Brân the Blessed and Brânwen. The first element in his name is cognate with the stem of the name of the Irish sea god Manannán mac Lir, and likely originated from the same Celtic deity as Manannán. Unlike Manannán, however, no surviving material connects him with the sea in any way except for his patronymic. Manawydan's most important appearances occur in the Second and Third Branches of the Mabinogi, but he is also referenced frequently in medieval poetry and the Welsh Triads.

Welsh mythology consists of both folk traditions developed in Wales, and traditions developed by the Celtic Britons elsewhere before the end of the first millennium. As in most of the predominantly oral societies Celtic mythology and history were recorded orally by specialists such as druids. This oral record has been lost or altered as a result of outside contact and invasion over the years. Much of this altered mythology and history is preserved in medieval Welsh manuscripts, which include the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin and the Book of Taliesin. Other works connected to Welsh mythology include the ninth-century Latin historical compilation Historia Brittonum and Geoffrey of Monmouth's twelfth-century Latin chronicle Historia Regum Britanniae, as well as later folklore, such as the materials collected in The Welsh Fairy Book by William Jenkyn Thomas (1908).