Mattapany-Sewall Archeological Site is an archaeological site in St. Mary's County, Maryland. It is located at the Patuxent River Naval Air Station on a level terrace approximately 45' above sea level, less than 1000' south of the Patuxent River in an unused wooded/grassy tract. Documentary evidence identifies the site as Mattapany-Sewall, a manor established in 1663 and occupied from 1666 to 1684 by Charles Calvert, 3rd Baron Baltimore.