The Fort Constitution State Historic Site is a historic area and state park in New Castle, New Hampshire. It is located overlooking the Pistaqua River.

Fort Constitution State Historic Site preserves and celebrates the location of
Fort William and Mary, a former colonial defensive post that was first established in 1632. It served as the only military post in the Portsmouth area for more than a century after its completion and served numerous purposes in its time.

Given to the state of New Hampshire in 1961, it was turned consequently into the Fort Constitution State Historic Site, a state park. It was later listed on the National Register of Historic Places. Today, it is open to the public as a recreation area.