In Greek mythology, Phineus (Ancient Greek: Φινεύς, ) was a king of Thrace and seer who appears in accounts of Argonauts' voyage. Several different versions of Phineus's parentage were presented in ancient texts. According to Apollonius of Rhodes, he was a son of Agenor, but the Bibliotheca says that other authors named his father as Poseidon. The Hesiodic Catalogue of Women, on the other hand, reported that Phineus was the son of Phoenix and Cassiopeia. His first wife was Cleopatra, daughter of Boreas and Oreithyia, by whom he had a pair of sons; his second wife Idaea, daughter of Dardanus, deceived him into blinding these sons, a fate Phineus himself would suffer.

Phineus's own blinding was variously attributed to the outrage against his sons, his giving Phrixus directions on his journey,, or because he preferred long life to sight. But the most famous reason for his disability is reported in the Argonautica: Zeus blinded Phineus for revealing the future to mankind. For this reason he was also tormented by the Harpies, who stole or defiled whatever food he had at hand or, according to the Catalogue of Women, drove Phineus himself to the corners of the world. However the Harpies plagued him, deliverance from this curse motivated Phineus's involvement in the voyage of the Argo.

When the ship landed by his Thracian home, Phineus described his torment to the crew and told them that his brothers-in-law, the wing-footed Boreads, both Argonauts, were fated to deliver him from the Harpies. Zetes demured, fearing the wrath of the gods should they deliver Phineus from divine punishment, but the old seer assured him that he and his brother Calais would face no retribution. A trap was set: Phineus sat down to a meal with the Boreads standing guard, and as soon as he touched his food the Harpies swept down, devoured the food and flew off. The Boreads gave chase, pursuing the Harpies as far as the "Floating Islands" before Iris stopped them lest they kill the Harpies against the will of the gods. She swore an oath by the Styx that the Harpies would no longer harangue Phineus, and the Boreads then turned back to return to the Argonauts; it is for this reason, according to Apollonius, that the "Floating Islands" are now called the Strophades, the "Turning Islands". Phineus then revealed to the Argonauts the path their journey would take and informed them how to pass the Symplegades safely, thus filling the same role for Jason that Circe did for Odysseus in the Odyssey.

A now lost play about Phineus, Phineus, was written by Aeschylus and was the first play in the trilogy that included The Persians, produced in 472 B.C.