Thermopolis is the county seat and most populous town in Hot Springs County, Wyoming, United States. As of the 2020 U.S. Census, the town population was 2,725. Thermopolis, Greek for "hot city", is home to numerous natural hot springs, in which mineral-laden waters are heated by geothermal processes. The town is named for the hot springs located there. The town claims the world's largest mineral hot spring, appropriately named "The Big Spring", as part of Hot Springs State Park.