The Bronx () is a borough of New York City, coextensive with Bronx County, in the U.S. state of New York, the third-most-densely populated county in the United States. It is south of Westchester County; northeast and east of Manhattan, across the Harlem River; and north of Queens, across the East River. The Bronx has a land area of 42 square miles (109 km2) and a population of 1,418,207 in 2019. Of the five boroughs, it has the fourth-largest area, fourth-highest population, and third-highest population density. It is the only borough predominantly on the U.S. mainland. If each borough were its own city, the Bronx would rank as the eighth-most-populous in the U.S.
The Bronx is divided by the Bronx River into a hillier section in the west, and a flatter eastern section. East and west street names are divided by Jerome Avenue. The West Bronx was annexed to New York City in 1874, and the areas east of the Bronx River in 1895. Bronx County was separated from New York County in 1914. About a quarter of the Bronx's area is open space, 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.