Hydra, or Ydra or Idra (EE-dra; Greek: Ύδρα, romanized: Ýdra, pronounced [ˈiðra] in Modern Greek), and in antiquity Hydrea, 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 (Ὑδρέα, 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 (area 49.6 km² (19.2 sq mi)), Dokos (area 13.5 km² (5.2 sq mi)), and a few uninhabited islets, with a total area of 64.443 km² (24.9 sq mi). The province of Hydra (Greek: Επαρχία Ύδρας) was one of the provinces of the Argolis and Corinthia Prefecture from 1833 to 1942, of Attica prefecture from 1942 to 1964, of Piraeus prefecture from 1964 to 1972, and of Attica again as part of the newly established Piraeus prefecture of Attica. Its territory corresponded with that of the current municipality. It was abolished in 2006. Today the municipality of Hydra is part of the Islands regional unit of Attica region.

There is one main town, known simply as "Hydra port" (population 1,900 as of 2011^[update]). It consists of a crescent-shaped harbor, around which is centered a strand of restaurants, shops, markets, and galleries that cater to tourists and to locals (Hydriots or Hydriotes). 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. Other small villages or hamlets on the island include Mandraki, Kamini, Vlychos, Palamidas, Episkopi, and Molos. Wheeled vehicles are not permitted on the island, including but not limited to cars, motorcycles, scooters, bicycles, and e-bikes.

The name Hydra comes from ancient Greek ὕδρα (hydra), derived from the Greek word for "water", a reference to the natural springs on the island. The local Arvanite name is attested in two variants: the common variant Nύδρα/Nidhra and the rare form Nidhriza or Hydriza.

Hydra depends on tourism, and Athenians account for a sizable segment of its visitors. High-speed hydrofoils and catamarans from Piraeus, some 37 nautical miles (69 km) away, serve Hydra, stopping first at Poros before going on to Spetses. There is a passenger ferry service providing an alternative to hydrofoils that operates between Hydra Harbor to Metochi on the Peloponnese coast. Many Athenians drive to Metochi, leave their car in the secure car park, and take the 20-minute passenger ferry across to Hydra.

Rubbish trucks are the only motor vehicles on the island, since by law, cars and motorcycles are not allowed. Horses, mules and donkeys, and water taxis provide public transportation. The inhabited area, however, is so compact that most people walk everywhere.

In 2007, a National Geographic Traveler panel of 522 experts rated Hydra the highest of any Greek island (11th out of 111 islands worldwide), as a unique destination preserving its "integrity of place".

The Tsamadou mansion, on the left side as one enters the harbor, is now a Maritime Academy. The Tsamados family donated the mansion for the purpose of hosting the Greek Maritime Academy on their island. Another house, originally built for the Tsamados family on the hill overlooking the port was donated to the church to be used as a weaving school and production facility of textiles and carpets. The house now belongs to the Colloredo-Mansfeld family and became known as the Old Carpet Factory.