Penobscot Bay (French: Baie de Penobscot) is an inlet of the Gulf of Maine and Atlantic Ocean in south central Maine, a stretch known as Midcoast Maine, in a broader Atlantic region known as Down East. The bay originates from the mouth of Maine's Penobscot River, downriver from Belfast. Penobscot Bay has many working waterfronts including Rockland, Rockport, Stonington, and Belfast. Penobscot Bay is between Muscongus Bay and Blue Hill Bay, just west of Acadia National Park.
At the beginning of the Holocene epoch 11,000 years ago, the Gulf of Maine's sea level fell as low as 180 feet (55 m) below its present height. Penobscot Bay was then a continuation of Penobscot River that meandered through a broad lowland extending past present day Matinicus Island.