The Mystic River is a  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 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, as the modern spelling has it, 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.