North Sioux City is a city in the far southeastern part of the U.S. state of South Dakota. It is located in Union County, near the junction of the Missouri River and the Big Sioux River. North Sioux City is near the borders of Iowa and Nebraska, and it is a suburb of Sioux City, Iowa. It became a city in 1951, and 2,530 people lived there at the 2010 census.