Governors Island is a 172 acres (70 ha) island in Upper New York Bay, approximately one-half mile (1 km) from the southern tip of Manhattan Island and separated from Brooklyn by Buttermilk Channel. It is legally part of the borough of Manhattan in New York City. The island was expanded by approximately 82 acres (33 ha) of landfill on its southern side when the Lexington Avenue subway was excavated in the early 1900s.

First named by the Dutch explorer Adriaen Block, it was called Noten Eylant (and later in pidgin language Nutten Island) from 1611 to 1784. The island's current name—made official eight years after the 1776 Declaration of Independence—stems from British colonial times when the colonial assembly reserved the island for the exclusive use of New York's royal governors.

Defensive works were raised on the island in 1776 by Continental Army troops during the American Revolutionary War, and fired upon British ships before falling into enemy hands. From 1783 to 1966, the island was a United States Army post. From 1966 to 1996 the island served as a major United States Coast Guard installation.

On January 19, 2001, Fort Jay and Castle Williams, two of the island's three historical fortifications were proclaimed a National Monument. On January 31, 2003, 150 acres of the island was transferred to the State of New York for a nominal fee of $1. The remaining (22 acres or 9 ha) was transferred to the United States Department of the Interior as the Governors Island National Monument, administered by the National Park Service.