The Bronx is the northernmost of the Five Boroughs of New York City and the newest of the 62 counties of New York State. It is located northeast of Manhattan and south of Westchester County. The Bronx is the only borough situated primarily on the North American mainland (while the other four are on islands, except for Marble Hill, a small portion of Manhattan.). In 2009, the U.S. Census Bureau estimated that the borough's population on July 1, 2008 was 1,391,903,[1] living on 42 square miles (109 square kilometers) of land, making the Bronx fourth of the five boroughs in population, fourth in area, and third in density of population.

The Bronx is divided by the Bronx River into a hillier section in the west, closer to Manhattan, and the flatter East Bronx, closer to Queens and Long Island. The West Bronx was annexed to New York City (then largely confined to Manhattan) in 1874, and the areas east of the Bronx River in 1895.[4] The Bronx first assumed a distinct legal identity when it became a borough of Greater New York in 1898. Bronx County (the County of Bronx), with the same boundaries as the borough, was separated from New York County (today coextensive with the Borough of Manhattan) in 1912 and began its own operations in January 1914.[5] Although the Bronx is the third-most-densely-populated county in the U.S.,[3] about a quarter of its land is open space,[6] including Woodlawn Cemetery, Van Cortlandt Park, Pelham Bay Park, the New York Botanical Garden and the Bronx Zoo in the borough's north and center, on land deliberately preserved in the late 19th century as urban development progressed northwards and eastwards from Manhattan with roads, bridges and railroads.