Fort Knox, now Fort Knox State Park or Fort Knox State Historic Site, is located on the western bank of the Penobscot River in the town of Prospect, Maine, about 5 miles (8.0 km) from the mouth of the river. Built between 1844 and 1869, it was the first fort in Maine built entirely of granite; most previous forts used wood, earth, and stone. It is named after Major General Henry Knox, the first U.S. Secretary of War and Commander of Artillery during the American Revolutionary War, who at the end of his life lived not far away in Thomaston. As a virtually intact example of a mid-19th century granite coastal fortification, it was added to the National Register of Historic Places in 1969 and declared a National Historic Landmark on December 30, 1970. Fort Knox also serves as the entry site for the observation tower of the Penobscot Narrows Bridge that opened to the public in 2007.

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).