The Pequannock River is a tributary of the Pompton River, approximately 20 miles (32 km) long, located in northern New Jersey in the United States. It rises in eastern Sussex County, near Highland Lakes on the north side of Hamburg Mountain. It flows southeast to Stockholm, then east-southeast, in a gap north of Green Pond Mountain and other ridges, past Butler and Bloomingdale. It joins the Ramapo River at Pompton Plains in Pequannock Township to form the Pompton. In its upper course, it forms part of the county line between Passaic and Morris counties.