Hart Island, sometimes referred to as Hart's Island, is located at the western end of Long Island Sound, in the northeastern Bronx in New York City. Measuring approximately 1 mile (1.6 km) long by 0.33 miles (0.53 km) wide, Hart Island is part of the Pelham Islands archipelago and is east of City Island.

The island's first public use was as a training ground for the United States Colored Troops in 1864. Since then, Hart Island has been the location of a Union Civil War prison camp, a psychiatric institution, a tuberculosis sanatorium, a potter's field used for both individual and mass burials, a homeless shelter, a boys' reformatory and workhouse, a jail, and a drug rehabilitation center. Several other structures, such as an amusement park, were planned for Hart Island but not built. During the Cold War, Nike defense missiles were stationed on Hart Island. The island was intermittently used as a prison and a homeless shelter until 1967; the last inhabited structures were abandoned in 1977. The potter's field on Hart Island was run by the New York City Department of Correction until 2019, when the New York City Council voted to transfer jurisdiction to the New York City Department of Parks and Recreation.

The remains of more than one million people are buried on Hart Island. Since the first decade of the 21st century, however, there have been fewer than 1,500 burials a year. Burials on Hart Island include individuals who were not claimed by their families or did not have private funerals; the homeless and the indigent; and mass burials of disease victims. Access to the island was restricted by the Department of Correction, which operated an infrequent ferryboat service and imposed strict visitation quotas. Burials were conducted by inmates from the nearby Rikers Island jail until 2020 and the COVID-19 pandemic. The Hart Island Project, a public charity founded by visual artist Melinda Hunt, worked to improve access to the island and make burial records more easily available. Transfer to the Parks Department in 2019 had been sought for over twenty years and was hoped to ease public access to the Island. Burials in the island's Potters' Field continued after the transfer.

There are numerous theories about the origins of the island's place name. One theory posits that British cartographers named it "Heart Island" in 1775 due to its organ-like shape but the 'e' was dropped shortly after. A map drawn in 1777 and subsequent maps refer to the island as "Hart Island". Other names given to the island during the late 18th century were "Little Minneford Island" and "Spectacle Island", the latter because the island's shape was thought to resemble spectacles.

Another theory, based on the meaning of the English word "hart", which means "stag", is that the island was named when it was used as a game reserve. Another version holds that it was named in reference to deer that migrated from the mainland during periods when ice covered that part of Long Island Sound.

Hart Island is approximately 1 mile (1.6 km) long by 0.33 miles (0.53 km) wide at its widest point. It lies about 0.33 miles (0.53 km) off the eastern shore of City Island. The island's area is disputed; according to some sources, it is 101 acres (41 ha), while others state that it is 131 acres (53 ha). Hart Island is isolated from the rest of the city: there is no electricity and the only means of access is via ferryboat.

Before European colonization, Hart Island was occupied by the Siwanoy tribe of Native Americans, who were indigenous to the area. In 1654, English physician Thomas Pell purchased the island from the Siwanoy as part of a 9,166-acre (37.09 km²) property. Pell died in 1669 and ownership passed to his nephew Sir John Pell, the son of British mathematician John Pell. The island remained in the Pell family until 1774, when it was sold to Oliver De Lancey. It was later sold to the Rodman, Haight, and Hunter families, in that order. According to Elliott Gorn, Hart Island had become "a favorite pugilistic hideaway" by the early 19th century. Bouts of bare-knuckle boxing held on the island could draw thousands of spectators.

The first public use of Hart Island was training the 31st Infantry Regiment of the United States Colored Troops beginning in 1864. A steamboat called John Romer shuttled recruits to the island from the Battery at the southern tip of Manhattan. A commander's house and a recruits' barracks were built; the barracks included a library and a concert room; it could house 2,000 to 3,000 recruits at a time, and over 50,000 men were ultimately trained there.