Fort O'Brien State Historic Site, also known as Fort Machias, preserves the remains of a fort located in Machiasport, Maine that was built and destroyed three times over a 90-year period. It was involved in military actions during the American Revolutionary War and the War of 1812, and was listed on the National Register of Historic Places. It is now managed by the Maine Department of Conservation's Bureau of Parks and Lands, and is open between Memorial Day and Labor Day.