In Greek mythology, Medusa (Μεδουσα "The Guardian"), was the only mortal of the three Gorgon sisters.

The gorgons, Medusa, Stheno, and Euryale, were female monsters with brass hands, sharp fangs and hair of living, venomous serpents. Every creature who looked directly at Medusa was turned to stone.

Medusa and her other sisters the Graeae were daughters of Phorcys and Ceto. Medusa was a beautiful maiden raped by Poseidon in Athena's temple. Poseidon was an arch-rival of Athena's since he vied for dominance over Athens; Athena offering the olive tree, Poseidon, the horse. Athena changed Medusa into a Gorgon as punishment for desecrating her temple. She turned Medusa's hair into snakes, her face hideous and banished her beyond the Hyperborean lands.

While Medusa was with child by Poseidon, she was killed by Perseus with help from Athena and Hermes. Perseus used Medusa's head to petrify Atlas, rescue Andromeda, and kill Polydectes. Then he gave it to Athena, who placed it on her shield Aegis.

From Medusa's blood came two offspring by Poseidon: Pegasus and Chrysaor.

Asteroid 149 Medusa, and an important geological region on the planet Mars, Medusa Fossae bear her name.