Southern Maryland National Heritage Area Act This bill establishes the Southern Maryland National Heritage Area in Maryland. The heritage area shall include portions of St. Mary's, Calvert, Charles, and Prince George's Counties in Maryland, with the specific boundaries to be established by the Department of the Interior, based on the feasibility study for the heritage area. The Tri-County Council for Southern Maryland shall serve as the local coordinating entity for the heritage area. The council shall submit a management plan for the heritage area.