The smallest national park in the United States is Gateway Arch NP in St. Louis, Missouri. This park is only 192 acres. Hot Springs National Park is the 2nd smallest National Park in the United States with only 5,549 acres which is 8.6 square miles of land. The 3rd smallest national park site is the National Park of American Samoa with only 8,256 acres. The smallest overall site in the National Park Service is Thaddeus Kosciuszko National Memorial in Pennsylvania that is only 0.02 acres.

Source: Park Ranger John