It lies between Manhattan Island to its west and the borough of Queens, on Long Island, to its east. It is politically part of the borough of Manhattan, and New York County. Running from the equivalent of East 46th to 85th Streets on Manhattan Island, it is about 2 miles (3.2 km) long, with a maximum width of 800 feet (240 m), and a total area of 147 acres (0.59 km²).
Together with Mill Rock, Roosevelt Island constitutes Manhattan's Census Tract 238, which has a land area of 0.279 sq mi (0.72 km²), and had a population of 9,520 as of the 2000 United States Census. It had a population of 11,661 as of the 2010 United States Census.