South Brooklyn is a historic term for a section of the former City of Brooklyn – now the New York City borough of Brooklyn – encompassing what are now the Boerum Hill, Carroll Gardens, Cobble Hill, Gowanus, Park Slope, Windsor Terrace, Sunset Park and Red Hook neighborhoods. It was named for its location along the waterfront that was the southern border of the original Village of Brooklyn, and has remained widely used as a colloquialism despite it no longer being the southernmost point of the borough. It should not be confused with the geographic southern region of the modern borough of Brooklyn, which includes the neighborhoods of Gravesend, Seagate, Coney Island, Brighton Beach, Manhattan Beach, Sheepshead Bay, Gerritsen Beach, Marine Park, Mill Basin, and Bergen Beach.

Brooklyn is a borough of New York City. Located on the westernmost end of Long Island, it is coextensive with Kings County in the U.S. state of New York. With 2,736,074 residents as of the 2020 United States census, Kings County is the most populous of the five boroughs of New York City and the most populous county in the State of New York. The population density of Brooklyn was 37,339.9 inhabitants per square mile (14,417.0/km2) in 2022, making it the second-most-densely-populated county in the United States, behind Manhattan, and it had the ninth-highest population of any county nationwide. Were Brooklyn still an independent city, it would be the fourth most populous in the U.S. after the rest of New York City, Los Angeles, and Chicago.