The Lernaean Hydra or Hydra of Lerna, more often known simply as the Hydra, is a serpentine lake monster in Greek mythology 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. In the canonical Hydra myth, the monster is killed by Heracles (Hercules) as the second of his Twelve Labors.

In Greek mythology, Eurystheus was king of Tiryns, one of three Mycenaean strongholds in the Argolid, although other authors including Homer and Euripides cast him as ruler of Argos.