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 of granite (instead of wood). 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. The fort was designated a National Historic Landmark in 1970, as a virtually intact example of a mid-19th century granite coastal fortification. Fort Knox also serves as the entry site for the observation tower of the Penobscot Narrows Bridge that opened to the public on May 19, 2007.
Fort Knox never saw battle, though it was manned during times of war. During the American Civil War, volunteers from Maine, mostly recruits in training before assignment to active duty, manned the fort. Thomas Lincoln Casey supervised work on the fort, including adapting the batteries to use the recently invented Rodman cannon, and oversaw its completion.
A regiment from Connecticut manned Fort Knox during the Spanish–American War. The garrison was reduced to one man, the "Keeper of the Fort", upon their departure at the end of the war. The keeper attended to the condition and maintenance of the fort, and reported to Fort Preble, in Portland.
In 1923, the federal government declared the fort excess property and put its 125-acre (51 ha) grounds up for sale. The state of Maine bought it for $2,121. It is now a Maine state historic site and was declared a U.S. National Historic Landmark on December 30, 1970.