| State: | SD |
| Type: | US |
| Route: | 18 |
| Map Custom: | yes |
| Map Notes: | US 18 highlighted in red |
| Length Mi: | 451.88 |
| Length Ref: | [1] |
| Established: | 1926 |
| Direction A: | West |
| Terminus A: | at the Wyoming state line near Edgemont |
| Junction: |
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| Direction B: | East |
| Terminus B: | at the Iowa state line near Canton |
| Counties: | Fall River, Oglala Lakota, Bennett, Todd, Tripp, Gregory, Charles Mix, Douglas, Hutchinson, Turner, Lincoln |
| Previous Type: | SD |
| Previous Route: | 17 |
| Next Type: | SD |
| Next Route: | 18 |
U.S. Highway 18 (US 18) is a part of the United States Numbered Highway System that travels from Orin, Wyoming, to Milwaukee, Wisconsin. In the state of South Dakota, US 18 runs from the Wyoming border east to the Iowa border. The highway runs parallel to the state's southern border with Nebraska, and at one point, near Fairfax, US 18 comes within one mile to the Nebraska border.
US 18 enters South Dakota west of Edgemont. It passes through Hot Springs, the Pine Ridge Indian Reservation, the Rosebud Indian Reservation, Winner, and Gregory, before crossing the Missouri River near Pickstown over Fort Randall Dam. East of the Missouri River, US 18 passes through (or near) Lake Andes and Tripp before a brief concurrency with Interstate 29 (I-29) near Worthing. East of I-29, US 18 passes through Canton before crossing the Big Sioux River into Iowa.[2]
The South Dakota section of US 18 is legally defined at South Dakota Codified Laws ยง 31-4-141.[3]