Twin Falls County is a county in the south central part of the U.S. state of Idaho. As of the 2020 census, the population was 90,046, making it the fifth-most populous county in Idaho. The county seat and largest city is Twin Falls. Twin Falls County is part of the Twin Falls, ID Twin Falls Metropolitan Statistical Area.

Cassia County is a county in the U.S. state of Idaho. As of the 2020 Census the county had a population of 24,655. The county seat and largest city is Burley. Cassia County is included in the Burley, ID Micropolitan Statistical Area.