Monona County is a county located in the U.S. state of Iowa. The population was 9,243 in the 2010 census, a decline from 10,020 in the 2000 census. The county seat is Onawa. Monona County was formed in 1851. It was perhaps named for the Indian word meaning "beautiful valley." The first Monona County Courthouse was completed in 1858.
Onawa is a city in Monona County. The population was 2,998 at the 2010 census. It is the county seat of Monona County. It is the largest town on the Iowa side of the Missouri River between Council Bluffs and Sioux City. The city has the widest main street in the United States - about 75 feet wide. Onawa was platted in 1857. The railroad was built through Onawa in 1867. Onawa was the site of a POW camp for captured German soldiers, between 1944 and 1946. Historical documents indicate there were never more than 50 POWs in camp. A larger camp existed near the central Iowa town of Algona, and housed as many as 5,400 German POWs.