Dallas County is a county located in the U.S. state of Iowa. As of the 2020 census, its population was 99,678, and was estimated to be 118,457 in 2025. making it the sixth-most populous county in Iowa. Between 2010 and 2020, it was the fastest growing county in Iowa and one of the fastest growing in the country. The county seat is Adel and the largest city is Waukee. The county was named for George M. Dallas, Vice President of the United States under James K. Polk, the namesake of neighboring Polk County.