In Greek mythology, Idas (/ˈaɪdəs/; Ancient Greek: Ἴδας, romanized: Ídas), was a Messenian prince. He was one of the Argonauts, a participant in the hunt for the Calydonian Boar and contender with the gods. Idas was described as keen and spirited.

Idas was the son of Aphareus and Arene and the elder brother of Lynceus and Pisus. He was sometimes regarded as the offspring of Poseidon. In some accounts, the wife of Aphareus and thus, Idas' possible mother was named as Polydora or Laocoosa. By Marpessa, Idas had one daughter named Cleopatra Alcyone who married Meleager.

When Idas came from Messenia to ask for the hand of Marpessa, daughter of Evenus. The maiden's father refused his request because he wanted his daughter to remain a virgin. Idas went to his father Poseidon and begged for the use of a winged chariot. Poseidon consented to his use of the chariot, and Idas stole Marpessa away from a band of dancers and fled to Pleuron in Aetolia. Her father, after chasing the couple for a long time and realizing he could not catch up to them, killed his horses and then drowned himself in a nearby river Lycormas and became immortal. The river was named later after him.

Apollo also pursued them in his own chariot, wanting Marpessa for himself.

Idas that was mightiest of men that were then upon the face of earth;
who also took his bow to face the king Phoebus Apollo
for the sake of the fair-ankled maid [i.e. Marpessa].

As the two fought for the girl's hand, Zeus eventually intervened and commanded Marpessa to choose between her mortal lover and the god. Marpessa chose Idas, reasoning to Apollo that had she chosen the god, she would have eventually grown old and lost his affections:

And thou beautiful god, in that far time,
When in thy setting sweet thou gazest down
On this grey head, wilt thou remember then
That once I pleased thee, that I once was young?

The two beautiful daughters of Leucippus, Phoebe and Hilaeira were promised brides of their cousins, Idas and Lynceus. Because of their beauty, the twins Castor and Pollux who were inflamed with love, carried off the maidens. Trying to recover their lost brides-to-be, the two Messenian princes, took to arms and joined the celebrated fight between them and their rival suitors. During the battle, Castor killed Lynceus while Idas, at his brother's death, forgot both the strife and bride, and started to bury his brother. When he was placing the bones in a funeral monument, Castor intervened and tried to prevent his raising of the monument, because he had won over him as if he were a woman. In anger, Idas pierced the thigh of Castor with the sword he wore. Others say that, as he was building the monument he pushed it on Castor and thus killed him. When they reported this to Pollux, he rushed up and overcame Idas in a single fight, recovered the body of his brother, and buried it.