Arkansas' Hot Springs National Park is the smallest fully fledged national park in the United States. This park, which includes commercial soaking facilities where visitors can enjoy the hot springs firsthand, covers about 5,500 acres. By contrast, several of Alaska's national parks, which are among the largest in the system, cover more than 3.5 million acres each.
According to the National Park Service, as of 2015 the smallest site in the National Park System is Pennsylvania's Thaddeus Kosciuszko National Memorial, which occupies just .2 acres. However, this is subject to change as the National Park System is always expanding and revising.