The Hackensack River is a river, approximately 45 miles (72 km) long, in the U.S. states of New York and New Jersey, emptying into Newark Bay, a back chamber of New York Harbor. The watershed of the river includes part of the suburban area outside New York City just west of the lower Hudson River, which it roughly parallels, separated from it by the New Jersey Palisades. It also flows through and drains the New Jersey Meadowlands. The lower river, which is navigable as far as the city of Hackensack, is heavily industrialized and forms a commercial extension of Newark Bay.

Once believed to be among the most polluted watercourses in the United States, it staged a modest revival by the late 2000s. The river is divided into the upper river, north of the Oradell Reservoir and Oradell Dam, and lower river, south of the reservoir and dam.