Thoosa enters Greek literature in the opening book of Homer's Odyssey. Athena is explaining why Odysseus, the Greek hero trying to return home after the Trojan War, is still being kept from Ithaca. Poseidon is angry because Odysseus blinded Polyphemus, the Cyclops who trapped the hero and his companions inside a cave and ate several of them. Homer then gives the monster's parentage: Polyphemus is the son of Poseidon and the nymph Thoosa, daughter of Phorcys, and Poseidon lay with her in hollow or arching caves.

Thoosa's father Phorcys belongs to an older generation of sea powers. Hesiod makes him a child of Pontus, the primordial sea, and Gaia, the Earth. Phorcys and the sea goddess Ceto produce several frightening beings, including the Graeae and the Gorgons. Homer, however, names only Phorcys as Thoosa's father and gives no mother for her. For that reason Ceto should not be inserted as Thoosa's mother simply because she is Phorcys's usual consort.

Thoosa is also not securely one of the Nereids, the fifty daughters of Nereus and Doris who appear as a separate family of sea nymphs. Homer calls Thoosa a nymph and identifies her through Phorcys. The distinction matters because her lineage ties Polyphemus to the ancient and sometimes monstrous sea powers rather than to the well-defined Nereid sisterhood. Ancient texts give Thoosa no serpent tail, claws or monstrous face. Her appearance is never described.

Her union with Poseidon produces Polyphemus, but Thoosa herself does not enter the Cyclops episode of the Odyssey. After Odysseus and his men escape by clinging beneath the Cyclops's sheep, Odysseus reveals his true name from the departing ship. Polyphemus then stretches his hands toward the sky and prays to Poseidon. He asks that Odysseus never reach home, or that he return only after long suffering, alone and in a foreign ship. Poseidon hears the prayer and becomes the divine force behind the disasters that delay Odysseus's return.

Thoosa reappears indirectly in Theocritus's eleventh Idyll, a Hellenistic poem that turns Polyphemus from a cannibalistic monster into a lovesick shepherd. The Cyclops sings to Galatea, a sea nymph he desires but cannot win. He recalls first seeing her when she came with his mother to gather flowers on the hillside. Later he complains that Thoosa never speaks to Galatea on his behalf, imagining that his mother should be able to repair the disappointment of an unreturned love.

Those few references contain almost everything securely ancient about Thoosa. Apollodorus repeats that Polyphemus is the son of Poseidon and the nymph Thoosa, and later writers preserve the same genealogy without giving her an independent myth. She has no recorded husband beyond her union with Poseidon, no other securely attested children, no cult, no death and no transformation. Ancient art gives Polyphemus many scenes, especially the blinding and his love of Galatea, but no surviving image can be securely identified as a central portrait of Thoosa.

Thoosa therefore matters through connection rather than action. Through Phorcys she links Polyphemus with the sea's archaic powers. Through Poseidon she gives the Cyclops direct access to an Olympian father. That parentage changes the consequences of Odysseus's victory. Blinding Polyphemus is not merely the defeat of an isolated monster. It creates a family grievance that the ruler of the sea answers across the rest of the Odyssey.