Thermopolis is the largest town in and the county seat of Hot Springs County, Wyoming, United States. As of the 2000 census, the town population was 3,172. As might be expected from its name and the name of its county, Thermopolis is home to numerous natural hot springs, in which mineral-laden waters are heated by geothermal processes. It claims the world's largest mineral hot spring as part of Hot Springs State Park. The springs are open to the public for free as part of an 1896 treaty signed with the Shoshone and Arapaho Indian tribes.