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 classical Greece, Lerna was a region of springs and a former lake located in the municipality of the same name, near the east coast of the Peloponnesus, south of Argos. Even though much of the area is marshy, Lerna is located on a geographically narrow point between mountains and the sea, along an ancient route from the Argolid to the southern Peloponnese; this location may have resulted in the importance of the settlement.