Thetis is a sea nymph or the goddess of water, one of the fifty Nereids, daughters of the ancient sea god Nereus.

When described as a Nereid in Classical myths, Thetis was the daughter of Nereus and Doris, and a granddaughter of Tethys with whom she sometimes shares characteristics. Often she seems to lead the Nereids as they attend to her tasks. Sometimes she also is identified with Metis.

Some sources argue that she was one of the earliest of deities worshipped in Archaic Greece, the oral traditions and records of which are lost. Only one written record, a fragment, exists attesting to her worship and an early Alcman hymn exists that identifies Thetis as the creator of the universe. Worship of Thetis as the goddess is documented to have persisted in some regions by historical writers such as Pausanias.

In the Trojan War cycle of myth, the wedding of Thetis and the Greek hero Peleus is one of the precipitating events in the war, leading also to the birth of their child Achilles.

Most extant material about Thetis concerns her role as mother of Achilles, but there is some evidence that as the sea-goddess she played a more central role in the religious beliefs and practices of Archaic Greece.