The Fomorians or Fomori are a supernatural race in Irish mythology, who are often portrayed as hostile and monstrous beings. Originally they were said to come from under the sea or the earth. Later, they were portrayed as sea raiders and giants. They are enemies of Ireland's first settlers and opponents of the Tuatha Dé Danann, the other supernatural race in Irish mythology; although some members of the two races have offspring. The Tuath Dé defeat the Fomorians in the Battle of Mag Tuired. This has been likened to other Indo-European myths of a war between gods, such as the Æsir and Vanir in Norse mythology and the Olympians and Titans in Greek mythology.

Irish mythology is the body of myths indigenous to the island of Ireland. It was originally passed down orally in the prehistoric era. In the early medieval era, some myths were transcribed by Christian monks, who heavily altered and Christianised the myths. Irish mythology is the best-preserved branch of Celtic mythology.