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. Prior to the 1860s, the Twin Falls County area was an unorganized part of Shoshone lands, then as part of the Oregon Country.