Nantucket Sound is a roughly triangular area of the Atlantic Ocean offshore from the U.S. state of Massachusetts. It is 30 miles (48 km) long and 25 miles (40 km) wide, and is enclosed by Cape Cod on the north, Nantucket on the south, and Martha's Vineyard on the west. Between Cape Cod and Martha's Vineyard it is connected to the Vineyard Sound.