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.