McCook County is a county in the U.S. state of South Dakota. At the 2020 census, the population was 5,682. Its county seat is Salem. The county was established in 1873, and was organized in 1878. It was named for the former governor of the Dakota Territory and Civil War general Edwin Stanton McCook.

Sioux Falls is the most populous city in the U.S. state of South Dakota and the 121st-most populous city in the United States. It is the county seat of Minnehaha County and also extends into northern Lincoln County to the south, which continues up to the Iowa state line. The population was 192,517 at the 2020 census, and in 2022, its estimated population was 202,078. According to city officials, the estimated population had grown to 213,891 as of early 2024. The Sioux Falls metro area accounts for more than 30% of the state's population. Chartered in 1856 on the banks of the Big Sioux River, the city is situated in the rolling hills at the junction of interstates 29 and 90.