Onawa is a city in Monona County, Iowa, United States. The population was 3,091 at the 2000 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's main claim to fame is that it has the widest main street in the United States - about 75 feet (23 m) wide.