Hudson River, the longest river in New York and a major river in the eastern United States. It flows 306 miles (492 km) southward from the Adirondack Mountains to New York City, where it enters Upper New York Bay, an arm of the Atlantic Ocean. The river is a source of hydroelectric power and has long been a major avenue of commerce.

In the Adirondacks the Hudson descends rapidly, following a scenic course with rapids and waterfalls. On leaving the mountains, near Hudson Falls, it begins a gentle flow to the sea. The river becomes a tidal estuary at Troy, where it is joined by its chief tributary, the Mohawk. Albany, a short distance south of Troy, is the head of navigation for oceangoing ships. The New York State Barge Canal system connects the Hudson in the Troy-Albany area with Lakes Erie, Ontario, and Champlain.

South of Albany, with the Catskill Mountains to the west, the Hudson passes Kingston, Poughkeepsie, and Newburgh and a number of large estates. Among them are the Roosevelt and Vanderbilt estates at Hyde Park. Between Newburgh and Peekskill the Hudson flows through a deep gorge in the Hudson Highlands, passing the U.S. Military Academy at West Point. There the river attains its greatest depth, about 200 feet (60 m). South of Peekskill it widens into a lake-like expanse called the Tappan Zee.

The final stretch—sometimes called the North River—forms the New York-New Jersey boundary. On the New Jersey side rise the high cliffs called the Palisades. Nearly a score of cities, including New York City, Yonkers, and Jersey City, adjoin the river. Crossing it are the George Washington Bridge, the Lincoln and Holland tunnels, and a number of railway tubes. Most of the port facilities of New York and New Jersey are in the bay beyond the river's mouth.

The Hudson was first sighted by Giovanni da Verrazano in 1524 and was explored by Henry Hudson in 1609. It provided an important route in colonial times and was of strategic importance in the Revolutionary War. Beginning with the opening of the Erie Canal in 1825, the Hudson became part of a major route across the Appalachians.