Welsh mythology consists of both folk traditions developed in Wales, and traditions developed by Britons elsewhere before the end of the first millennium. Like most predominately oral societies found in Pre-Roman Great Britain, Welsh mythology and history was recorded orally by Druids (derwydd). This oral record has been lost or altered as result of outside contact and invasion over the years. Much of this altered mythology and history are 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 9th century Latin historical compilation Historia Britonum (the History of the Britons) and Geoffrey of Monmouth's 12th-century Latin chronicle Historia Regum Britanniae (the History of the Kings of Britain), as well as later folklore, such as The Welsh Fairy Book by W. Jenkyn Thomas [1908].

Celtic mythology is the mythology of Celtic polytheism, the religion of the Iron Age Celts. Like other Iron Age Europeans, the early Celts maintained a polytheistic mythology and religious structure. Among Celts in close contact with Ancient Rome, such as the Gauls and Celtiberians, their mythology did not survive the Roman Empire, their subsequent conversion to Christianity, and the loss of their Celtic languages. It is mostly through contemporary Roman and Christian sources that their mythology has been preserved. The Celtic peoples who maintained either their political or linguistic identities (such as the Gaels in Ireland, and the Brittonic tribes of Great Britain) left vestigial remnants of their ancestral mythologies, put into written form during the Middle Ages.