Hydra 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 the narrow Hydra Gulf.

Hydra is served by high speed hydrofoils and catamarans from Piraeus plus daily island tour boats.

Besides garbage trucks, motor vehicles are not permitted on the island, which leaves the bulk of public transportation up to horses, donkeys and water taxis. The inhabited area, however, is so compact that most people walk everywhere.

There is one main town, known simply as "Hydra port". It consists of a crescent-shaped harbor, around which is centered a strand of commercial establishments (restaurants, shops, markets, and galleries), all of which cater to tourists and locals. Steep stone streets lead up and outwards from the harbor area.

Most of the local residences, as well as the guest accommodation on the island are located on these streets. Other small villages or hamlets on the island include Mandraki, Kamini, Vlychos, Palomitha, Episkopi, and Molos.

Although the island's name is derived from ancient springs, it is now almost dry.