Hydra was an ironclad warship of the Greek Navy, named for Hydra, one of the Saronic Gulf islands which played a key role in the war at sea during the Greek War of Independence. The lead ship of her class of ironclads, she was ordered in 1885 in response to a crisis in the Balkans and Ottoman naval expansion. The ship was launched in 1889 and delivered to Greece by 1892. She was armed with a main battery of three 10.8 in (270 mm) guns and five 5.9 in (150 mm) guns, and had a top speed of 17 kn.

The Hellenic Navy is the naval force of Greece, part of the Hellenic Armed Forces. The modern Greek navy historically hails from the naval forces of various Aegean Islands, which fought in the Greek War of Independence. During the periods of monarchy it was known as the Royal Hellenic Navy.