The Oyster River is a 17-mile-long (27 km) river in Strafford County, southeastern New Hampshire, United States. It rises in Barrington, flows southeast to Lee, then east-southeast in a serpentine course past Durham to meet the entrance of Great Bay into Little Bay. The bays are tidal inlets of the Atlantic Ocean, to which they are connected by a tidal estuary, the Piscataqua River. The freshwater portion of the river is 14.1 miles (22.7 km) long, and the tidal river extends 2.9 miles (4.7 km) from Durham to Great Bay.