The Penobscot River is a 109-mile-long (175 km) river in the U.S. state of Maine. Including the river's West Branch and South Branch increases the Penobscot's length to 264 miles (425 km), making it the second-longest river system in Maine and the longest entirely in the state. Its drainage basin contains 8,610 square miles (22,300 km2).

Penobscot Indian Island Reservation is an Indian reservation for the Penobscot Tribe of Maine, a federally recognized tribe of the Penobscot in Penobscot County, Maine, United States, near Old Town. The population was 758 at the 2020 census. The reservation extends for many miles alongside 15 towns and two unorganized territories in a thin string along the Penobscot River, from its base at Indian Island, near Old Town and Milford, northward to the vicinity of East Millinocket, almost entirely in Penobscot County. A small, uninhabited part of the reservation used as a game preserve and hunting and gathering ground is in South Aroostook, Aroostook County, by which it passes along its way northward.