It rises in a mountainous area of central Orange County, New York, near Monroe and flows southeast to Harriman, then south into western Rockland County, then into northern Bergen County, New Jersey. In New Jersey, it flows SSW, along the east side of the ridge of the Ramapo Mountains. It flows into Potash Lake in Oakland and officially ends at Pompton Lake in Pompton Lakes. The river that flows out of Pompton Lake is still called the Ramapo River until it forms a confluence with the Pequannock River; from then on it is called the Pompton River. A portion of the river's water is diverted to the nearby Wanaque Reservoir.
The river is a popular destination for fly fishing of trout. New York State Route 17, the Port Jervis commuter railroad line, and the New York State Thruway were all constructed in the valley of the river between Harriman and Suffern, New York.
Areas noted along the Ramapo River were inhabited by bands of the Lenape tribe, a large group who inhabited much of the coastal areas of the mid-Atlantic region. Some of their people retreated to the mountains under pressure from Dutch and English colonists. The Lenape occupied western Long Island, the areas of present-day metropolitan New York, northern and southern New Jersey and the Delaware River Valley, south through Pennsylvania into Delaware.
Many smaller archeological sites in Oakland, New Jersey have been found to have been inhabited by prehistoric indigenous peoples. Areas of Oakland also contain many areas where fossilized crustaceans can be found along the banks of the river.
Descendants of the Lenape include the Ramapough Lenape Indian Tribe (also known as the Ramapough Mountain Indians, recognized in 1980 by the state of New Jersey. Many have lived in Mahwah and Ringwood, New Jersey and Hillburn, New York since the late colonial period. They also intermarried with people of other ethnicities.
- EPA: Ramapo Aquifer Systems
- United States Military Academy: Ramapo River Watershed Model and Environmental Study
- Torne Valley Preservation Association
- U.S. Geological Survey: NJ stream gaging stations