South Sioux City is a city in Dakota County, Nebraska, United States. It is located immediately across the Missouri River from Sioux City, Iowa, …South Sioux City is a city in Dakota County, Nebraska, United States. It is located immediately across the Missouri River from Sioux City, Iowa, and is part of the Sioux City, IA-NE-SD Metropolitan Statistical Area. As of the 2010 census, the city population was 13,353, making it the 14th most populous city in Nebraska.