Hydra (Greek: Ύδρα, pronounced [ˈiðra] in modern Greek) is one of the Saronic Islands of Greece, located in the Aegean Sea between the Saronic Gulf and the Argolic Gulf. It is separated from the Peloponnese by narrow strip of water.

In ancient times, the island was known as Hydrea (Υδρέα, derived from the Greek word for water), which was a reference to the springs on the island.

The dominant geographic features of Hydra are its rocky hillsides, which are bare, pine forested valleys with the occasional farmhouse. The island was subject to a modern geologic study by Renz in 1955. Some of the later Permian limestone strata are rich in well-preserved fossils.