Block Island Sound is a strait in the open Atlantic Ocean, approximately 10 miles (16 km) wide, separating Block Island from the coast of mainland Rhode Island in the United States. On the west, it extends to Montauk Point on the eastern tip of Long Island, as well as Plum Island, Gardiners Island, and Fishers Island, all in the state of New York.Geographically, Block Island Sound is the eastward extension of Long Island Sound as well as the westward extension of Rhode Island Sound.