Ellis Island, at the mouth of the New York Harbor, was at one time the main entry facility for immigrants entering the United States from January 1, 1892 until November 12, 1954. It is wholly in the possession of the Federal government as a part of Statue of Liberty National Monument and is under the jurisdiction of the US National Park Service. It is situated in New York City and Jersey City, New Jersey.

Ellis Island was the subject of a border dispute between New York State and New Jersey (see below). According to the United States Census Bureau, the island, which was largely artificially created through the landfill process, has an official land area of 129,619 square meters, or 32 acres, more than 83 percent of which lies in the city of Jersey City. The natural portion of the island, lying in New York City, is 21,458 square meters (5.3 acres), and is completely surrounded by the artificially created portion. For New York State tax purposes it is assessed as Manhattan Block 1, Lot 201. Since 1998, it also has a tax number assigned by the state of New Jersey.