Des Moines Listen/dɨˈmɔɪn/ is the capital and the most populous city in the US state of Iowa. It is also the county seat of Polk County. A small portion of the city extends into Warren County. It was incorporated on September 22, 1851, as Fort Des Moines which was shortened to "Des Moines" in 1857. It is named after the Des Moines River, which may have been adapted from the French Rivière des Moines, literally meaning "River of the Monks.