Poweshiek County is a county in the southeastern part of the U.S. state of Iowa. As of the 2020 census, the population was 18,662. The county seat is Montezuma. The county is named for the chief of the Fox tribe who signed the treaty ending the Black Hawk War. It lies along Interstate 80 between Des Moines and Iowa City. Poweshiek County's largest city is Grinnell.