Mill Rock Island is a small unpopulated island between Manhattan and Queens in New York City, in the U.S. state of New York. It lies about off Manhattan's East 96th Street, south of Randall's and Ward's Island where the East River and Harlem River converge. The island forms Census Block 9000 of Census Tract 238 in New York County. (Except for Mill Rock Island, Census Tract 238 is entirely comprised of Roosevelt Island.) Its official area is 16,173 square meters, or .
This area was infamous as a treacherous area for shipping vessels to pass, and was known as the Hell Gate.
In 1701, John Marsh built a mill there that gave the island its name. The island was later squatted on by Sandy Gibson, who operated a farm on the island. At that time there were two islands, Great and Little Mill Islands.
It was used as an American fort during the War of 1812, where the War Department built a blockhouse with two cannons on Great Mill Rock. This fortification was part of a chain of blockhouses that was intended to defend New York Harbor and protect the passage into Long Island Sound from the British Navy.
In 1885, the United States Army Corps of Engineers detonated 300,000 lb (136,000 kg) of explosives on adjoining Flood Rock; that island had been the most treacherous impediment to East River shipping. It was, most likely, the most forceful explosion in New York City's history at the time; it was felt as far away as Princeton, New Jersey. The Flood Island remnants were used to fill the space between Great and Little Mill Islands, producing Mill Rock.