In Greek mythology, Thetis (Greek: Θέτις) is a sea nymph, one of the fifty Nereids, the daughters of Nereus, the Old Man of the Sea, and Doris. Like other sea figures, Thetis bore the gift of prophecy as well as the ability to change her shape at will. She was married to the mortal hero Peleus, by whom she is the mother of Achilles.