The South Branch Penobscot River is a river in Somerset County, Maine. Its source, Penobscot lake, the north end of which at is about from the Canada – United States border in Sandy Bay (Maine Township 5, Range 3, NBKP). This section of the border runs along the height of land separating the watersheds of the Penobscot River and the Monument River, which feeds into the Saint Lawrence River. The South Branch of the Penobscot River runs northeast through Canada Falls Lake to its drowned confluence with the North Branch of the Penobscot in Seboomook Lake in Pittston Academy Grant (T.2 R.4 NBKP). The two rivers combine to form the West Branch Penobscot River, which flows east from the outlet of Seboomook Lake.