The Portland Observatory is a historic maritime signal tower at 138 Congress Street in the Munjoy Hill section of Portland, Maine. Built in 1807, it is the only known surviving tower of its type in the United States. Using both a telescope and signal flags, two-way communication between ship and shore was possible several hours before an incoming vessel reached the docks. The tower was designated a National Historic Landmark in 2006; it is now managed by a local nonprofit as a museum, and is open to the public.