Grant County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 7,556. The county seat is Milbank. The county was founded in 1873 and organized in 1878. It is named for Ulysses S. Grant, 18th President of the United States.

Big Stone Lake is a long, narrow freshwater lake and reservoir on the border between western Minnesota and northeastern South Dakota in the United States.