The Lernaean Hydra or Hydra of Lerna, more often known simply as the Hydra, is a serpentine water monster in Greek and Roman mythology. Its lair was the lake of Lerna in the Argolid, which was also the site of the myth of the Danaïdes. Lerna was reputed to be an entrance to the Underworld, and archaeology has established it as a sacred site older than Mycenaean Argos.

According to Hesiod, the Hydra was the offspring of Typhon and Echidna. The Hydra possessed many heads, the exact number of which varies according to the source. Later versions of the Hydra story add a regeneration feature to the monster: for every head chopped off, the Hydra would regrow two heads.