Sioux City is a city in Woodbury and Plymouth counties in the northwestern part of the U.S. state of Iowa. The population was 85,797 in the 2020 census, making it the fourth-most populous city in Iowa. The bulk of the city is in Woodbury County, of which it is the county seat, though a small northern portion is in Plymouth County. Sioux City is located at the navigational head of the Missouri River.