The Mystic River Reservation is a publicly owned nature preserve with recreational features located along the Mystic River in the towns of Winchester, Arlington, Medford, Somerville, Everett, and Chelsea in eastern Massachusetts. The reserve is part of the nearly 76-square-mile (200 km2) Mystic River watershed. It is managed by the Massachusetts Department of Conservation and Recreation.

The Mystic River is a 7.0-mile-long (11.3 km) river in Massachusetts. In the Massachusett language, missi-tuk means "large estuary", alluding to the tidal nature of the Mystic River. The resemblance to the English word mystic is a coincidence, which the colonists followed.