In Greek mythology, Thetis is a sea goddess, grand daughter of Tethys, and the mother of Achilles by Peleus, king of the Myrmidons. Poseidon and Zeus were both interested in her, but a prophecy ordained that her son would be greater than his father, and so they made arrangements for her marriage to a mortal man. Peleus, son of AEacus , courted her, but she refused him. He then found her when she was asleep and bound her tightly to keep her from escaping by changing form. She then consented to marry him, though without love or interest.

The wedding of Thetis and Peleus was celebrated on Olympus and attended by all the deities. However, Eris, the goddess of discord, was not invited. In spite, she threw a golden apple into the midst of the goddesses that was to be awarded only to the fairest.

Thetis had several children by Peleus, but she killed them all by casting them into a fire to test whether or not they were immortal. All failed, and Achilles would have done so as well, except that Peleus took the boy from her grasp.

Later, Thetis tried to make Achilles invulnerable by dipping him in the waters of the Styx (the river of Hades). However, the heel by which she held him was not protected by the Styx's waters. In the story of the Achilles in the Trojan War in Iliad, Homer does not mention Achilles's weakness.

In an earlier and less popular version of the story, Thetis anointed the boy in ambrosia and put him on top of a fire to burn away the mortal parts of his body. She was interrupted by Peleus and abandoned both father and son in a rage. Peleus gave him to Chiron to raise.

Prophecy said that her son would have either a long but dull life or a glorious but brief life. When the Trojan War broke out, Thetis was anxious and concealed him in the court of lycomedes. He went with the rest of the Greeks anyway. Thetis then had Vulcan make a suit of armor but then refused to pay him the favors she promised for the armor.

When Achilles was killed by Paris, Thetis came from the sea with the Nereides to mourn him, and she collected his ashes in a golden urn and raised a monument to his memory and instituted festivals.

Thetis was one of the gods behind the Oracle at Delphi, which she received from Gaia and gave to Phoebe.