Jason (Ancient Greek: Ἰάσων, Iásōn) was an ancient Greek mythological hero who was famous for his role as the leader of the Argonauts and their quest for the Golden Fleece. He was the son of Aeson, the rightful king of Iolcus. He was married to the sorceress Medea.

Jason appeared in various literature in the classical world of Greece and Rome, including the epic poem Argonautica and tragedy, Medea. In the modern world, Jason has emerged as a character in various adaptations of his myths, such as the 1963 film Jason and the Argonauts and the 2000 TV miniseries of the same name.

Jason has connections outside of the classical world, as he is seen as being the mythical founder of the city of Ljubljana, the capital of Slovenia.

Jason's father is invariably Aeson, but there is great variation as to his mother's name. According to various authors, she could be:

Jason was also said to have had a younger brother Promachus and a sister Hippolyte, who married Acastus (see Astydameia).

Pelias (Aeson's half-brother) was very power-hungry, and he wished to gain dominion over all of Thessaly. Pelias was the product of a union between their shared mother, Tyro ("high born Tyro") the daughter of Salmoneus, and allegedly the sea god Poseidon. In a bitter feud, he overthrew Aeson (the rightful king), killing all the descendants of Aeson that he could. He spared his half-brother for unknown reasons. Alcimede I (wife of Aeson) already had an infant son named Jason whom she saved from being killed by Pelias, by having women cluster around the newborn and cry as if he were still-born. Alcimede sent her son to the centaur Chiron for education, for fear that Pelias would kill him — she claimed that she had been having an affair with him all along. Pelias, still fearful that he would one day be overthrown, consulted an oracle which warned him to beware of a man with one sandal.