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. Ports on Nantucket Sound include Nantucket and Hyannis, Massachusetts.
Nantucket Sound possesses significant marine habitat for a diversity of ecologically and economically important species. "The Sound" has particular significance for several federally protected species of wildlife and a variety of commercially and recreationally valuable fisheries.
The Sound is located at a confluence of the cold Labrador Currents and the warm Gulf Stream. This creates a unique coastal habitat representing the southern range for Northern Atlantic species and the northern range for Mid-Atlantic species. Nantucket Sound has much biological diversity and contains habitats that range from open sea to salt marshes, as well as warm-water beaches on the Cape and Islands coasts.