river in Maine, U.S., formed by several headstreams draining numerous lakes that were created by melting glaciers. It is the state's longest river, about 350 miles (560 km) in length. Its western and eastern branches join at Medway and run in a southeasterly and then southwesterly direction to empty into the Atlantic Ocean via Penobscot Bay near Bucksport. The river's major tributary is the Mattawamkeag. Bangor, 23 miles (37 km) from the ocean, is the head of navigation. The river's former economic importance as a source of salmon was superseded by the development of its hydropower facilities. Penobscot Bay, actually an estuary, extends 35 miles (56 km) inland and is 27 miles (43 km) wide at its mouth; it includes many islands and sheltered harbours, and tourism is significant.

Navigated by English voyagers in 1603 and by Samuel de Champlain (Champlain, Samuel de) in 1604, the river was named for the Penobscot Indians. Its valley became a bloody battleground for the French and British between 1673 and 1759 and for the British and Americans until 1815.