Hydra, or Ydra or Idra is one of the Saronic Islands of Greece, located in the Aegean Sea between the Myrtoan Sea and the Argolic Gulf. It is separated from the Peloponnese by a narrow strip of water. In ancient times, the island was known as Hydrea, derived from the Greek word for "water"), a reference to the natural springs on the island.

The municipality of Hydra consists of the islands Hydra, Dokos, and a few uninhabited islets. The province of Hydra was one of the provinces of the Argolis and Corinthia prefecture from 1833 to 1942, Attica prefecture from 1942 to 1964, Piraeus prefecture from 1964 to 1972 and then back to Attica as part of the newly establishment Piraeus prefecture of Attica prefecture. Its territory corresponded with that of the current municipality. It was abolished in 2006. Today the municipality of Hydra is part of Islands regional unit of Attica region.

There is one main town, known simply as "Hydra port". It consists of a crescent-shaped harbor, around which is centered a strand of restaurants, shops, markets, and galleries that cater to tourists and locals (Hydriots). Steep stone streets lead up and outward from the harbor area. Most of the local residences, as well as the hostelries on the island, are located on these streets.