The river forms at the confluence of the North and South Branches just west of Somerville at the border of Bridgewater, Branchburg, and Hillsborough Townships. It flows for approximately 16 mi (25.7 km) before slowing in tidewater at New Brunswick, and its estuary extends 14 mi (22.5 km) more entering the western end of Raritan Bay at South Amboy.
The river has served an important water transportation route since the Pre-Columbian era. The name Raritan is also applied to the Raritan people, an Algonquian tribe that inhabited Staten Island, near the river's mouth. In colonial days, the river allowed the development of early industry around New Brunswick, as well as the transportation of agricultural materials from central New Jersey. During the American Revolutionary War, the river provided a means for troop conveyance. The construction of the Delaware and Raritan Canal along the right (south) bank of the river provided a critical link between New York City and Philadelphia, Pennsylvania on the Delaware River.