The Rattlesnake River is a 3.6 mi river in eastern New Hampshire in the United States. It is a tributary of the Cocheco River, part of the Piscataqua River watershed leading to the Atlantic Ocean. The river is located entirely in the town of Farmington. It rises north of Hussey Mountain and Chesley Mountain and flows east to the Cocheco, dropping nearly 500 ft in elevation over its length.