Sanborn County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 2,330. Its county seat and largest city is Woonsocket. The county was created by the Dakota Territorial legislature on May 1, 1883, with land partitioned from Miner County. It was fully organized by July 18, 1883.

South Dakota Highway 34 is a state route that runs parallel to Interstate 90 across the entire state of South Dakota. It begins at the Wyoming border west of Belle Fourche, as a continuation of Wyoming Highway 24 (WYO 24). The eastern terminus is at the Minnesota border east of Egan, or southwest of Airlie, Minnesota, where it continues as Minnesota State Highway 30 (MN 30). It is just over 419 miles (674 km) in length, making it the longest state highway in South Dakota.