(Gaelic: “The Shadowy One”), in Celtic mythology, female warrior, especially noted as a teacher of warriors.

Scáthach was the daughter of Árd-Greimne of Lethra. She lived on an island (thought to be the Isle of Skye) in an impregnable castle, the gate of which was guarded by her daughter Uathach. At this fortress Scáthach trained numerous Celtic heroes in the arts of pole vaulting (useful in the assault of forts), underwater fighting, and combat with a barbed harpoon of her own invention, the gáe bolg. Her best-known student was Cú Chulainn, who stayed with her for a year in order to learn the skills that helped him win many battles. A number of other heroes of Celtic mythology also owed their prowess to the training of the Amazon Scáthach.