Barnum Island is a hamlet and census-designated place in Nassau County, New York, United States. The population was 2,590 at the 2020 census. It occupies the eastern portion of an island situated between Long Island and Long Beach. That island, previously known in its entirety as Hog Island, consists entirely of the communities of Barnum Island and Island Park. Barnum Island is an unincorporated area of the Town of Hempstead.