Water Island is the fourth and most recently acquired main island in the U.S. Virgin Islands, a United States territory located in the Caribbean Sea. The island is of volcanic origin and lies to the south of Saint Thomas in the Charlotte Amalie harbor. Ferry service runs regularly from Crown Bay, Saint Thomas to Phillips Landing, Water Island; the ferry ride is about 10 minutes.

At 491.5 acres in size, Water Island is the smallest of the main U.S. Virgin Islands. It is administratively a part (subdistrict) of the St. Thomas District. Water Island is a residential island, with a population of 161 (2000 census) and no significant commercial establishments. A number of homes on Water Island are available to accommodate visitors. The main attractions are beaches, including Honeymoon Beach, plantation ruins, and Fort Segarra, an underground fort partially constructed by the U.S. during World War II. The northern end of the island is a gated community, Sprat Bay Estates, which includes Sprat Bay Beach. All of the beaches in the USVI are public.