Barren Island is a peninsula and former island on the southeast shore of Brooklyn in New York City. Located on Jamaica Bay, it was geographically part of the Outer Barrier island group on the South Shore of Long Island. The island was occupied by the Lenape Native Americans prior to the arrival of Dutch settlers in the 17th century. Its name is a corruption of Beeren Eylandt, the Dutch-language term for "Bears' Island".

Barren Island remained sparsely inhabited before the 19th century, mainly because of its relative isolation from the rest of the city. Starting in the 1850s, the island was developed as an industrial complex with fish rendering plants and other industries, and also as an ethnically diverse community of up to 1,500 residents. Between the mid-19th century and 1934, the island housed industrial plants that processed the carcasses of the city's dead horses, converting them into a variety of industrial products. This activity led to the still-extant waterbody on the island's western shore becoming nicknamed "Dead Horse Bay". A garbage incinerator, which became the subject of numerous complaints because of its odor, operated on the island from the 1890s to 1921.

The Barren Island community became known as South Flatlands during its final years. By the 1920s, most of the industrial activity had tapered off, and landfill was used to unite the island with the rest of Brooklyn. While most residents were evicted in the late 1920s for the construction of Floyd Bennett Field, some were permitted to stay until 1942, when the airfield was expanded as a wartime base of the United States Navy. No trace remains of the former island's industrial use. Since 1972, Floyd Bennett Field has been part of the Gateway National Recreation Area, managed by the National Park Service.

Barren Island was originally part of an estuary at the mouth of Jamaica Bay and acted as a barrier island for the larger Long Island, located to Barren Island's north. The bay, in turn, was created during the end of the Wisconsin glaciation. The edge of the glacier had been in the middle of Long Island, creating a series of hills across the island. The water from the melting glacier ran downhill toward a low-lying delta that adjoined the Atlantic Ocean, which later became Jamaica Bay. A body of water called Rockaway Inlet, located south of the island, connected the bay with the ocean. By the late 17th century, Barren Island had 70 acres (28 hectares) of salt meadows and 30 acres (12 ha) of uplands, as well as cedar forests. Salt, reed grasses, and hay served as food for early settlers' livestock.

Barren Island's geography was significantly altered by shifting tides and storms in the 19th century. Originally, it was part of the Outer Barrier, a series of barrier islands on Long Island's southern shore. Barren Island, the Rockaways, Pelican Beach, and Plumb Beach were separate barrier islands protecting Jamaica Bay. In an 1818 survey of Long Island, Barren Island was described as having dunes and scattered trees. By 1839, Barren Island, Plumb Beach, and Pelican Beach were a single island, separated from Coney Island to the west by Plumb Beach Inlet. By the end of the century, Gerritsen Inlet had formed, separating Barren Island from Plumb and Pelican Beaches.

The neighboring island of Rockaway Beach also had a large impact on Barren Island's geography. Originally, Rockaway was located to Barren's east, and the two islands' southern tips were aligned. From the mid-19th century, Rockaway Beach was extended more than one mile (1.6 km) to the southwest after several jetties were built to protect manmade developments there. This caused changes to Barren Island's ecology, because during the early 20th century, it had contained sand dunes on the coasts and salt marshes inland. As a result of the extension of Rockaway Beach, Barren Island was no longer a barrier island and its beach was washed away.

By the late 1920s, industrial development on Barren Island's eastern side had transformed that area, with a small patch of dunes remaining. The western side of the island, containing dunes, woods, and wetlands, was largely untouched. The southern coast contained a tidal creek stretching into the center of the island, where the tidal wetlands remained mostly intact. The wetlands were abutted by the human-made Mill Basin to the north. Tidal creeks also stretched across the island's marshes, although one of these creeks was bisected by the construction of Flatbush Avenue in 1925. In addition, the water adjoining the northern and western coasts had become heavily polluted. The entirety of the former island, covering 1,300 acres (530 hectares), was converted to a peninsula during the 1920s. It is occupied by Floyd Bennett Field, which in turn is part of the Gateway National Recreation Area.

The Barren Island area was originally the homeland of the Canarsee, a group of Lenape Native Americans, who referred to the archipelago of islands near it by a name alternatively transcribed as "Equandito", "Equendito", or "Equindito", which means "Broken Lands". This name also applied to several smaller islands in the area, such as Mill Island. Throughout its existence, Barren Island has also been referred to as "Broken Lands" in English, as well as "Bearn Island", "Barn Island", and "Bear's Island". The name "Barren Island" is a corruption of the Dutch Beeren Eylandt; it does not relate to the English word describing the geography of the island.