The Hudson River is a 315-mile (507 km) watercourse that flows from north to south through eastern New York State in the United States. The highest official source is at Lake Tear of the Clouds, on the slopes of Mount Marcy in the Adirondack Mountains. The river itself officially begins in Henderson Lake in Newcomb, New York. The river flows southward past the state capital at Albany and then eventually forms the boundary between New York City and the U.S. state of New Jersey at its mouth before emptying into Upper New York Bay. Its lower half is a tidal estuary, which occupies the Hudson Fjord. This formed during the most recent North American glaciation over the latter part of the Wisconsin Stage of the Last Glacial Maximum, 26,000 to 13,300 years ago. Tidal waters influence the Hudson's flow as far north as Troy, New York.

The river is named after Henry Hudson, an Englishman sailing for the Dutch East India Company, who explored it in 1609. It had previously been observed by Italian explorer Giovanni da Verrazano sailing for King Francis I of France in 1524, as he became the first European known to have entered the Upper Bay, but he considered the river to be an estuary. The Dutch called the river the "North River" – with the Delaware River called the "South River" – and it formed the spine of the Dutch colony of New Netherland. Settlement of the colony clustered around the Hudson, and its strategic importance as the gateway to the American interior led to years of competition between the English and the Dutch over control of the river and colony.