Cass County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 13,127. Its county seat is Atlantic. It was named to honor Lewis Cass, who was the 1848 Democratic nominee for president.

Atlantic is a city in and the county seat of Cass County, Iowa, United States, located along the East Nishnabotna River. The population was 6,792 in the 2020 census, a decline from the 7,257 population in 2000.