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.

Penobscot County is a county in the U.S. state of Maine, named for the Penobscot people in Wabanakik. As of the 2020 census, the population was 152,199, making it the third-most-populous county in Maine. Its county seat is Bangor. The county was established on February 15, 1816, from part of Hancock County when the area was still part of Massachusetts. Penobscot County is home to the University of Maine.