Mill Rock Island is a small island between Manhattan and Queens located south of Randall's and Ward's Island where the East and Harlem rivers converge. Renouned as a treacherous area for shipping vessals to pass, it was know as the Hellgate.

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 in fact two islands, Great and Little Mill Islands.

The island has a military history too, most notable for its use as an American fort during the War of 1812. In 1885 the US Army 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 is most likely the most forceful explosion in New York City history and was felt as far away as Princeton, NJ. The decimated Flood Island was used to fill the space between Great and Little Mill Islands, producing Mill Rock.

Between the 1960s and 1980s there were various plans to make this roughly 8.5 acres (34,000 m²) of vacant land in a park. Even though it is officially listed as a New York City park, nothing was done about this. The island is accessible only by boat.