Phineus enters Greek tradition with several competing genealogies. Apollonius of Rhodes calls him the son of Agenor and places him among the Thracians, while Hesiodic traditions connect him with Phoenix and Cassiopeia. His kingdom also shifts between Salmydessus on the Thracian coast and lands around Thynia and Bithynia. These variations do not change his central role. Phineus stands at the edge of the familiar Greek world as a ruler who possesses knowledge of the dangerous route farther east.

Apollo gives Phineus the gift of prophecy. In Apollonius' Argonautica, the seer uses that gift without sufficient restraint and reveals the will of Zeus too clearly to mortals. Zeus punishes him with blindness and a lingering old age. Other traditions give different causes. One says he revealed the road to Phrixus, the hero carried east by the Golden Ram. Another says he chose long life at the cost of sight. Still others connect his blindness with the cruel treatment of the sons of his first wife.

His family becomes part of the Argonautic rescue. Phineus marries Cleopatra, daughter of Boreas, the divine North Wind. Her brothers Zetes and Calais inherit wings from their father and later join Jason aboard the Argo. Phineus also takes a second wife, Idaea, in darker versions of the myth. She accuses her stepsons of attacking her, and Phineus believes the accusation, leading to imprisonment, mutilation or death depending on the source. These stories make the blind king both victim and offender rather than a simple innocent sufferer.

The Harpies turn his punishment into daily torment. Whenever food is set before Phineus, the winged creatures swoop down and seize it. What little remains is polluted by such a foul smell that no one can eat it. By the time the Argonauts arrive, Phineus is weak with hunger but recognises the heroes through prophecy. He promises to guide them if they free him from the creatures that make every meal impossible.

The Argonauts prepare another meal as bait. As soon as the Harpies descend, Zetes and Calais launch themselves into the air and chase them over the sea. In Apollonius, the pursuit reaches the Floating Islands, where Iris, divine messenger and relative of the Harpies, stops the Boreads and swears that the creatures will never trouble Phineus again. The heroes turn back, and the islands become the Strophades, the Turning Islands. Other traditions let the Harpies fall exhausted or die during the pursuit.

Freed from starvation, Phineus repays the Argonauts with the knowledge that makes the next stage of their voyage possible. He explains the coast ahead and warns them about the Symplegades, two rocks that smash together whenever a ship tries to pass. The Argonauts must first release a dove. If it survives the gap, they must row immediately behind it with all their strength. The advice works, and the Argo becomes the first ship to break through the deadly passage.

Phineus disappears differently in different traditions. Some later stories restore his sight through Asclepius or another healer, while hostile family traditions let Heracles or the Boreads kill him for the suffering of his sons. Greek tragedy also explored the figure, though much of that material is lost. The most influential surviving portrait remains Apollonius' blind, starving oracle: a man whose physical sight is gone but whose knowledge can guide heroes through waters he can no longer see.