According to the United States Census Bureau, the county has a total area of , of which is land and (1.1%) is water. 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 area and of water area. The geographies transferred are known now as Census Tract 14 in Gallatin County, and as Census Tract 6 in Park County.
