Brooklyn () is a borough of New York City, coextensive with Kings County, in the U.S. state of New York. Kings County is the most populous county in the State of New York, and the second-most densely populated county in the United States, behind New York County (Manhattan). Brooklyn is also New York City's most populous borough, with 2,736,074 residents in 2020.

Named after the Dutch town of Breukelen, Brooklyn is located on the westernmost edge of Long Island and shares a border with the borough of Queens. It has several bridge and tunnel connections to the borough of Manhattan, across the East River, and is connected to Staten Island by way of the Verrazzano-Narrows Bridge. With a land area of 70.82 square miles (183.4 km²) and a water area of 26 square miles (67 km²), Kings County is the state of New York's fourth-smallest county by land area and third-smallest by total area.

Brooklyn was founded by the Dutch in the 17th century and grew into a busy port city by the 19th century. On January 1, 1898, after a long political campaign and public relations battle during the 1890s, in accordance to the new Municipal Charter of "Greater New York", Brooklyn was consolidated in and annexed (along with other areas) to form the current five borough structure of New York City. The borough continues to maintain some distinct culture. Many Brooklyn neighborhoods are ethnic enclaves. Having a larger Jewish population than Jerusalem, the borough has been described as "the most Jewish spot on Earth", with Jews forming around a quarter of its population. Brooklyn's official motto, displayed on the Borough seal and flag, is Eendraght Maeckt Maght, which translates from early modern Dutch as "Unity makes strength."

In the first decades of the 21st century, Brooklyn has experienced a renaissance as a destination for hipsters, with concomitant gentrification, dramatic house price increases, and a decrease in housing affordability. Some new developments are required to include affordable housing units. Since the 2010s, parts of Brooklyn have evolved into a hub of entrepreneurship, high technology start-up firms, postmodern art and design.