The Mystic River is a 3.4-mile-long (5.5 km)estuary in the southeast corner of the U.S. state of Connecticut. Its main tributary is Whitford Brook. It empties into Fishers Island Sound, dividing the village of Mystic between the towns of Groton and Stonington. Much of the river is tidal. The location of three large, 19th-century shipbuilding firms, Mystic River is now the home of the Mystic Seaport maritime museum.