In Greek mythology, Pegasus, or Pegasos (Πήγασος) was a winged horse that was the foal of Poseidon, in his role as horse-god and the Gorgon Medusa.

Descriptions vary as to the winged stallion's birth and his brother the giant, Chrysaor; some say that they sprang from Medusa's neck as Perseus beheaded her, a "higher" birth, like the birth of Athena from the head of Zeus. Others says that they were born of the earth as Medusa's blood spilled onto it, in which case Poseidon would not be their sire. Minerva caught and tamed Pegasus, and presented him to the Muses.

The word Pegasos is derived from the Greek for the word spring, and everywhere the winged horse struck hoof to earth, an inspiring spring burst forth: one on the Muses' Mount Helicon, the Hippocrene ("horse spring"), at the behest of Poseidon to prevent the mountain swelling too much and another at Troezen.

Mounted on Pegasus, and with the Gorgon's head safely in his magical bag, Perseus was completed as a hero, and was ready to win Andromeda.

Pegasus aided the hero Bellerophon, who is a double in some way for Perseus, in his fight against both the Chimera and the Amazons. There are varying tales as to how Bellerophon found Pegasus, some say that the hero found him drinking at the Pierian spring and that Polyidus told Bellerophon how to find and tame him, others that either Athena or Poseidon brought him to Bellerophon.

Prior to aiding Bellerophon, Pegasus brought thunderbolts to Zeus, and following Bellerophon's death he returned to Mount Olympus to aid the gods. In his later life, Pegasus took a wife, Euippe (or Ocyrrhoe), by whom he had two children, Celeris and Melanippe. This family is the origin of the winged horses.

Pegasus was eventually turned into a constellation, but a single feather fell to the earth near the city of Tarsus (hence its name).