Brooklyn (/ˈbrʊklɪn/) is a borough of New York City, coextensive with Kings County, in the U.S. state of New York. It is the most populous county in the state, the second-most densely populated county in the United States, and New York City's most populous borough, with an estimated 2,648,403 residents in 2020. If each borough were ranked as a city, Brooklyn would rank as the third-most populous in the U.S., after Los Angeles and Chicago.

Named after the Dutch village of Breukelen, it shares a land border with the borough of Queens, at the western end of Long Island. Brooklyn has several bridge and tunnel connections to the borough of Manhattan across the East River, and the Verrazzano-Narrows Bridge connects it with Staten Island. 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 New York state's fourth-smallest county by land area, and third-smallest by total area.

Brooklyn was an independent incorporated city (and previously an authorized village and town within the provisions of the New York State Constitution) until January 1, 1898, when, after a long political campaign and public relations battle during the 1890s, according to the new Municipal Charter of "Greater New York", Brooklyn was consolidated with other cities, towns, and counties, to form the modern City of New York, surrounding the Upper New York Bay with five constituent boroughs. The borough continues, however, to maintain a distinct culture. Many Brooklyn neighborhoods are ethnic enclaves. 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, Brooklyn has evolved into a thriving hub of entrepreneurship, high technology start-up firms, postmodern art and design.