Sioux City is a city in Woodbury and Plymouth Counties, Iowa, United States. The population was 85,797 at the 2020 census, and was estimated at 86,875 in 2024, making it the fourth-most populous city in Iowa. The county seat of Woodbury County, Sioux City is the primary city of the five-county Sioux City metropolitan area, which had 144,334 residents on the 2020 census. Sioux City and the surrounding areas of northwestern Iowa, northeastern Nebraska and southeastern South Dakota are sometimes referred to collectively as Siouxland.