Thetis, in Greek mythology, a Nereid loved by Zeus and Poseidon. When Themis (goddess of Justice), however, revealed that Thetis was destined to bear a son who would be mightier than his father, the two gods gave her to Peleus, king of the Myrmidons of Thessaly. Thetis, unwilling to wed a mortal, resisted Peleus’s advances by changing herself into various shapes. But, assisted by the wise centaur Chiron, Peleus finally captured her.
All the gods brought gifts to their wedding. The child of their union was the warrior Achilles, but, according to some authorities, Thetis bore seven children, all of whom perished either when she attempted to render them immortal by fire or when she destroyed them as the tokens of an unwilling alliance. According to one story, Peleus stymied Thetis’s attempt to make Achilles immortal by appearing at the wrong moment, and she deserted him. She saved Zeus when Poseidon, Hera, and Athena revolted against him, and she rescued both Hephaestus and Dionysus from the sea. She had a sanctuary at Sparta.