According to the US Census Bureau, the county has an area of , of which is land and (0.4%) is water. The highest natural point in Montana, Granite Peak at 12,807 feet (3,904m), is in Park County. The county attained its present boundaries in 1978, when the former Yellowstone National Park (part) county-equivalent was dissolved and apportioned between Gallatin County and Park County. Gallatin County received of land area and of water area, whereas Park County received of land and of water. The geographies transferred are known as Census Tract 14 in Gallatin County and Census Tract 6 in Park County.