Delaware River, a river in the eastern United States, about 400 miles (640 km) long. From its source in the Catskill Mountains in southeastern New York the river flows southeastward to form part of the borders between Pennsylvania and New York, Pennsylvania and New Jersey, and New Jersey and Delaware. It empties into Delaware Bay.

Near Stroudsburg, Pennsylvania, in the Appalachian Mountains, the river flows through the Delaware Water Gap, a gorge about two miles (3 km) long. The beautiful scenery of the gorge, a national recreation area, attracts many tourists.

A major industrial area lies along the lower course of the Delaware River. This area includes the cities of Philadelphia, Pennsylvania; Trenton and Camden, New Jersey; and Wilmington, Delaware. Ships can sail upstream as far as Trenton.

The river drains an area of about 12,000 square miles (31,000 km2). The main tributaries are the Musconetcong (New Jersey), the Schuylkill and Lehigh (Pennsylvania), and the Mongaup and Neversink (New York). The Chesapeake and Delaware Canal, a section of the Intracoastal Waterway, connects the Delaware River near its mouth with Chesapeake Bay in the west.