Sweetwater County is a county in southwestern Wyoming, United States. As of the 2020 United States Census, the population was 42,272, making it the fourth-most populous county in Wyoming. Its county seat is Green River. By area, it is the largest county in Wyoming. Its southern boundary line abuts the north lines of the states of Colorado and Utah.

Fremont County is a county in the U.S. state of Wyoming. As of the 2020 United States census, the population was 39,234, making it the fifth-most populous county in Wyoming. Its county seat is Lander. The county was founded in 1884 and is named for John C. Frémont, a general, explorer, and politician. It is roughly the size of the state of Vermont.