Although most of the surviving material concerning Thetis treats her role as the mother of Achilles, evidence suggests that she held considerably greater importance in Archaic Greek religious practice than her later, largely literary, role might imply. The pre-modern etymology of her name, from tithemi, 'to set up, establish,' has been read as a sign that the early Greeks perceived an ordering or foundational role for her.

After the death of Achilles, having earlier established a rapport with Zeus through her assistance in divine disputes, Thetis is said to have carried him away to the White Island (Leuke) in the Black Sea, an alternate Elysium where the hero transcended death and lived on.

Thetis and the other deities: Both Zeus and Poseidon courted Thetis, but a prophecy warned that her son would grow greater than his father. To avoid disrupting the divine order, the gods arranged her marriage to the mortal Peleus. When Hephaestus was cast from Olympus, the Oceanid Eurynome and the Nereid Thetis sheltered him on Lemnos, where he worked as their smith. According to Homer, when Dionysus was driven off by Lycurgus he took refuge with Thetis in a bed of seaweed. In one account Thetis and Medea competed for beauty, and when the Cretan Idomeneus judged Thetis the winner, Medea cursed all Cretans to be liars.

Marriage to Peleus: Peleus sought to marry Thetis, but she refused him. Following the advice of Proteus, Peleus found her sleeping and bound her fast, holding on as she shapeshifted into flame, water, a lioness, and a serpent. Subdued at last, she consented to marry him and bore Achilles.

The wedding was held on Mount Pelion outside the cave of Chiron and was attended by the gods, with feasting and celebration. Apollo played the lyre while the Muses sang. Chiron gave Peleus an ashen spear, its shaft polished by Athena and its blade forged by Hephaestus; other gifts included a bowl embossed with Eros from Aphrodite, a chlamys from Hera, a flute from Athena, and the immortal horses Balius and Xanthus from Poseidon. The uninvited Eris threw down the golden apple that, through the Judgement of Paris, would spark the Trojan War.

According to Apollonius of Rhodes's Argonautica, Thetis tried to make Achilles immortal by burning away his mortality each night while anointing him with ambrosia by day. When Peleus discovered this and cried out, Thetis flung the child down in anger and departed to the sea, never to return; some myths trace Achilles's vulnerable heel to this interruption. A variant tradition, notably in Statius's Achilleid, instead has Thetis dip Achilles in the River Styx to make him invulnerable, leaving only the heel by which she held him unprotected.

Peleus entrusted the boy to Chiron to be raised. When the Trojan War broke out, Thetis hid Achilles disguised as a girl at the court of Lycomedes, king of Scyros. After Calchas prophesied that the Greeks needed Achilles, Odysseus uncovered him by raising a false alarm, at which the young warrior instinctively seized weapons and revealed himself. Accepting that his destiny was inevitable, Thetis had Hephaestus forge his armor. According to Phylarchus, she agreed to lie with Hephaestus in exchange for the weapons, then fled his advances once armed; unable to pursue, he struck her ankle with his hammer, and she was healed in a Thessalian town afterward named Thetidium.

Iliad and the Trojan War: In Homer's Iliad, nine years into the war, Achilles quarrels with Agamemnon over Briseis. Feeling dishonored, Achilles prays to Thetis, who promises to petition Zeus; she persuades the god, who seals the agreement with a binding nod. After the death of Patroclus, Thetis consoles Achilles, promising new armor from Hephaestus and warning him not to arm himself until she returns; he obeys her even when Iris urges him to fight. Thetis obtains the splendid armor, and upon delivering it preserves Patroclus's body from decay by placing ambrosia and nectar in his nostrils. When Achilles, having killed Hector, keeps the body to desecrate it, the gods send Thetis to convey their anger, and she persuades her son to return the corpse for ransom.

Thetis was not widely venerated through cult practice, with one notable exception in conservative Laconia. Pausanias reported that priestesses of Thetis existed there in archaic times, centered on a wooden cult image that preceded the oldest temple; after the intervention of an influential woman her cult was re-established in a temple, where she was deeply revered into the second century AD.

A fragmentary hymn by the seventh-century BC Spartan poet Alcman depicts Thetis as a demiurge who initiates creation, bringing forth poros (path or track), tekmor (marker or end-post), and then darkness, the Sun, and the Moon. Scholars have noted close connections between Thetis and Metis, another shapeshifting sea-power bound by prophecy to bear a son who would surpass his father and later desired by Zeus.

Herodotus recorded that the Persians sacrificed to 'Thetis' at Cape Sepias, though this most likely reflects an interpretatio graeca, identifying a foreign sea-goddess (probably Anahita) with the Greek Thetis.

Thetis appears across ancient literature, including Homer's Iliad, Euripides's Andromache, Apollonius Rhodius's Argonautica, and the Bibliotheca. In later art, Francesco Cavalli composed the 1639 opera Le nozze di Teti e di Peleo on her wedding, and W. H. Auden's poem 'The Shield of Achilles' imagines her witnessing the forging of the shield. On screen, Maggie Smith played Thetis in the 1981 film Clash of the Titans, and Julie Christie portrayed her in the 2004 film Troy.