The Mystic River is a 3.4-mile-long 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, Connecticut between the towns of Groton and Stonington. Much of the river is tidal. The Mystic River was the location of three large shipbuilding firms during the 19th-century, and it is now the home of the Mystic Seaport maritime museum. The name Mystic is derived from the Pequot term "missi-tuk", describing a large river whose waters are driven into waves by tides or wind, according to the Mystic River Historical Society.
The Mystic River Bridge which carries the former Connecticut Turnpike (now Interstate 95) over the river is located towards the northern source of the river.