United States v. Maine, 469 U.S. 504 (1985), also known as the Rhode Island and New York Boundary Case, was a decision by the U.S. Supreme Court, which held that Long Island Sound and Block Island Sound in part constitute a juridical bay under Article 7(6) of the Convention on the Territorial Sea and the Contiguous Zone, Long Island being an extension of the mainland and the southern headland of the bay, and (b) that the bay closed at the line drawn from Montauk Point at the eastern tip of Long Island to Watch Hill Point on the Rhode Island shore, the waters of the bay west of the closing line being internal state waters, and the waters of Block Island Sound east of that line being territorial waters and high seas.

Maine is named in the title of the case because it is the northernmost of the thirteen defendant states with coastline on the Atlantic Ocean in a series of cases related to overlapping claims of state and federal jurisdiction over seas and the seafloor.

If it were, for the Convention's purposes, an island, then the sounds would be considered open waters and be under federal control.

The court ruled in favor of the states, determining that the East River, which separates Long Island from the mainland at the Bronx, was too small for modern navigation until humans altered it.

Long Island and the adjacent shore also share a common geological history.