Thermopolis is the largest town in Hot Springs County, Wyoming, United States, and also the county seat. As of the 2010 census, the town population was 3,009.
"Thermopolis" () is from the Greek for "Hot City". It 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 Wyoming's 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.
Dinosaur fossils were found on the Warm Springs Ranch in 1993, and the Wyoming Dinosaur Center was founded soon after.
In 1991, the Associated Press reported, in an article entitled "Hot Springs Rationed: Natural Wonder Drying Up:"
Thermopolis is located near the northern end of the Wind River Canyon and Wedding of the Waters, where the north-flowing Wind River becomes the Bighorn River. It is an unusual instance of a river changing names at a point other than a confluence of two streams. The dual name is ascribed to the mountain barrier between the Wind River and Bighorn basins, obscuring the fact that the river that drains the two is the same. The term "Wedding of the Waters" dates to at least 1934, when a marker was placed at the location.
Thermopolis is ringed by mountains, with the Big Horn Mountains to the northeast, the Bridger Mountains to the southeast, the Owl Creek Mountains to the southwest and the Absaroka Range to the northwest.