Brooklyn is the most populous of the five boroughs and counties in New York City, United States. Located at the westernmost end of Long Island, it shares a land border with the borough and county of Queens to the north, east, and south. The East River separates Brooklyn from Manhattan to the west and north, while the Verrazzano–Narrows Bridge connects it to Staten Island in the southwest. As Kings County, it serves as one of the original twelve counties established under British rule in 1683.

Founded by the Dutch in the 17th century, Brooklyn began as the small town of Breuckelen on the East River shore of Long Island. The name derives from the Dutch town of Breukelen, with the first appearance in print occurring in 1663. On January 1, 1898, after a prolonged political campaign, Brooklyn was consolidated and annexed to form the current five-borough structure of New York City. It retains a distinct culture despite its integration into the larger metropolis.

The borough features several bridge and tunnel connections to Manhattan, including the architecturally significant Brooklyn Bridge. Notable districts include Downtown Brooklyn, anchored by Fulton Street and Borough Hall, and Brooklyn Heights, known for its elegant brownstone buildings and historic churches. Prospect Park, a national historic site preferred by designer Frederick Law Olmsted, offers extensive public space. The area also includes diverse neighborhoods such as Williamsburg, Crown Heights, and Brighton Beach.

Brooklyn is described as one of the main global hubs for Jewish culture, with Jews forming around a fifth of its population. Many neighborhoods serve as ethnic enclaves, including areas with significant Russian and Asian communities. Cultural attractions include the Brooklyn Academy of Music, which boasts world-class theater performances, and the Barclays Center, home to the NBA's Brooklyn Nets. The borough maintains a distinct identity through its varied architecture and vibrant community life.

The population was 2,736,074 as of 2020, making it the most populous county in New York State. With a density of 37,339.9 inhabitants per square mile, it is the second most densely populated county in the U.S. after Manhattan. The borough has experienced a renaissance as a destination for entrepreneurship and high-technology startups. Transport access is provided by numerous subway and bus lines, along with major bridges connecting it to other parts of New York City.