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, Picts, and Brythonic tribes of Great Britain and Ireland) left vestigial remnants of their ancestral mythologies, put into written form during the Middle Ages.

Though the Celtic world at its apex covered much of western and central Europe, it was not politically unified nor was there any substantial central source of cultural influence or homogeneity; as a result, there was a great deal of variation in local practices of Celtic religion (although certain motifs, for example the god Lugh, appear to have diffused throughout the Celtic world). Inscriptions of more than three hundred deities, often equated with their Roman counterparts, have survived, but of these most appear to have been genii locorum, local or tribal gods, and few were widely worshipped. However, from what has survived of Celtic mythology, it is possible to discern commonalities which hint at a more unified pantheon than is often given credit.