South Dakota Highway 20 (SD 20) is a state highway in northern South Dakota, United States, that connects the Montana state line, west-southwest of Camp Crook, with the Minnesota state line, east-southeast of Revillo, via Buffalo, Bison, Timber Lake, Mobridge, Selby, and Watertown. From Mobridge to the Walworth–Potter county line, this highway is part of the Lewis and Clark Trail. From Timber Lake to just west of Mobridge, this highway is part of the Native American Scenic Byway. From the Montana state line to Camp Crook, the highway is a dirt road.

SD 20 is composed of two disconnected segments. The longer western segment extends from Montana to Watertown. The much-shorter eastern segment extends from west of South Shore to Minnesota. However, the South Dakota Department of Transportation previously indicated that SD 20 followed US 212, US 81, and Interstate 29 (I-29) between the two segments.

SD 20 originally extended from SD 45 west of Cresbard to Watertown. It was extended west to U.S. Route 83, southwest of Lowry. Later, it was extended to its current eastern terminus. Later still, it was extended to its current western terminus, replacing what had been SD 8 (and later SD 18).

Route description Harding County

SD 20 begins at an intersection with the eastern terminus of Tie Creek Road on the Montana state line west-southwest of Camp Crook, in the west-central part of Harding County. Here, the roadway continues to the south as Little Missouri Road. Although SD20 touches the Montana border, they and South Dakota are the only states to share a land border that is not traversed by a paved road.SD 20 travels to the north on the state line as a dirt road. Approximately later, it curves to a due-east direction. About four miles later it has a short curve northeast, then enters Camp Crook, where the highway's dirt portion ends. Just east of Park Avenue, it curves to the southeast and leaves Camp Crook. It crosses over the Little Missouri River, then turns east again and intersects the northern terminus of South Camp Crook Road, which leads to Sky Ranch and Belle Fourche. The highway curves to the east-northeast. SD 20 parallels a brief portion of Ash Creek. Farther to the east, it crosses over Graves Creek. Then, it parallels a brief portion of Brush Creek. It intersects the southern terminus of Clanton Road, which leads to Gardner Lake. Just before entering Buffalo, it crosses over Rush Creek. In the south-central part of the town, it intersects U.S. Route 85. The highways begin a concurrency to the south. Almost immediately, they leave Buffalo. They cross over the South Fork Grand River. When they split, SD 20 resumes its eastward direction. It curves to the east-southeast and crosses over Clarks Fork Creek. It intersects the northern terminus of Park Road, which leads to the Antelope Range and Livestock Research Station. It curves to the east-northeast and enters Custer National Forest. The highway travels through Reva Gap and passes Flag Rock. It curves to the northeast and leaves the forest. Just before curving to the east, it passes the location of the Battle of Slim Buttes, a battle fought between the United States Army and the Sioux. It then begins a concurrency with SD 79. This intersection is also the southern terminus of Tickle Hill Place. The two highways travel through Reva. They curve to the east-southeast and then split. SD 20 curves to the southeast and then back to the east. At an intersection with 155th Avenue, it enters the west-central part of Perkins County.

Perkins County SD 20 continues to the east and travels through Prairie City. Almost immediately, it crosses over the South Fork Thunder Butte Creek. Approximately east of Divide Road, it crosses over this creek again. At an intersection with the northern terminus of 170th Avenue, SD 20 meets the southern terminus of SD 75. The highway then enters the far northern part of Bison. The highway travels through rural areas of the county. It travels through Meadow and then intersects SD 73. The two highways travel concurrently to the south. When they split, they also met the eastern terminus of Rolling Hills Road. SD 20 continues to the east. It travels just north of Coal Springs. A little ways east of 200th Avenue, it enters the northeast part of both Ziebach County and the Cheyenne River Indian Reservation.

Ziebach County SD 20 curves to the southeast and then back to the east. It begins to curve back to the southeast just before traveling through Glad Valley. Just before Dorsey Road, it curves back to the east. It intersects SD 65 on the Ziebach–Dewey county line. The two highways travel concurrently to the east, on part of the county line. A short distance later, they enter Dewey County proper.

Dewey and Corson counties SD 20 and SD 65 continue to the east. They skirt along the southern edge of Isabel Municipal Airport and then curve to the north. They enter the city limits of Isabel. They curve to the northeast, where they split. SD 20 curves to the east and leaves Isabel. It curves to the north and enters Firesteel. It begins a curve to the east and leaves the community. It crosses the Little Moreau River, and almost immediately, intersects the southern terminus of County Road 243 (243rd Avenue) and SD 63. SD 20 and SD 63 begin to travel concurrently to the east. Just west of an intersection with 248th Avenue, they begin to curve to the east-northeast. They then enter Timber Lake. Almost immediately, they intersect the northern terminus of H Street, which leads to the Timber Lake Municipal Airport and the Little Moreau Recreation Area. In the north-central part of the city is an intersection with Main Street, which leads to the business district. They leave Timber Lake and curve to the north. They then curve to the east and begin to travel on the Dewey–Corson county line, which is also the dividing line between the Cheyenne River and Standing Rock Sioux reservations. They continue to the east through rural areas and then split, with SD 20 continuing to the east. Just west of the appropriately-named Lake Road, the highway travels just south of the Trail City Railroad Lake. It then travels through Trail City. SD 20 curves to the north-northeast, entering Corson County and Standing Rock Sioux Reservation proper, and curves back to the east. It curves to the northeast and then to the north. It curves to the north-northeast and intersects US 12. The two highways travel concurrently to the southeast. They curve to the northeast and cross over Claymore Creek. They intersect SD 1806, which joins the concurrency. The three highways continue to the northeast and curve to the east-southeast, where SD 1806 leaves to the north. US 12 and SD 20 head to the southeast, skirting along the southern edge of the Indian Memorial Recreation Area. They cross over part of the Missouri River known as Lake Oahe (signed as "Missouri River Lake Oahe") Here, they leave the Standing Rock Sioux Reservation and enter the northwestern part of Walworth County.

Walworth and Potter counties US 12 and SD 20 cross over some railroad tracks of BNSF Railway and curve to the south. During this curve, they enter Mobridge. Just south of 12th Street West, they curve to the southeast. Just south of 6th Street West, they curve to the east. At 4th Avenue East, SD 1804 joins the concurrency. It intersects Airport Road and Revheim Road, which lead to Mobridge Municipal Airport, the South Dakota National Guard armory, and the Revheim Park Recreation Area. The highways then leave Mobridge. A short distance later, SD 1804 departs to the south, while US 12 and SD 20 continue to the east. Just to the east of 293rd Avenue is a crossing of Blanket Creek. They enter the northern part of Glenham. Immediately, they have an intersection with the southern terminus of Glenham Road, which leads to Pollock. Just to the east of Main Street, they leave Glenham and continue to the east. Just east of 305th Avenue, they intersect US 83, which joins the concurrency. The three highways curve to the southeast and then to the south. They then enter Selby. At an intersection with Scranton Street, they meet the western terminus of SD 130. Just south of Walworth Street, they cross over some railroad tracks of BNSF Railway and pass some sewage disposal ponds. Then, they leave Selby. Just before an intersection with 134th Street, US 12 splits off to the southeast, while US 83 and SD 20 turn right to the southwest. They curve back to the south. Between 136th and 137th streets, they cross over Rieger Creek. Just south of 139th Street, they curve to the southwest. Just northeast of 306th Avenue, they curve to the south. They intersect the eastern terminus of SD 144 (142nd Street). Just north of 144th Street, they curve to the south-southeast and cross over Swan Creek. Just north of 147th Street, they curve back to the south. At 148th Street, on the Walworth–Potter county line, SD 20 splits off to the east, on the county line. Just east of 318th Avenue, it passes Hoven Municipal Airport. At 319th Avenue, SD 47 joins SD 20 in a concurrency to the east. Almost immediately, they enter Hoven. They pass a city park and then make a 90-degree curve to the south. They enter the main part of Hoven and Potter County proper. Between 3rd and 4th avenues, they pass a bus station. Between 5th and 6th avenues, they pass Hoven High School. South of 7th Avenue, they leave Hoven and continue to the south. At an intersection with the eastern terminus of CR 819 (151st Street), SD 20 splits off to the east. It travels just to the south of Tolstoy. At an intersection with 329th Avenue, it enters the northwestern part of Faulk County.

Faulk County SD 20 curves to the southeast and travels through the northeastern part of Onaka. It then curves back to the east. Between 340th and 343rd avenues, it passes Clark Lake. Between 348th and 349th avenues, the highway travels just north of Norbeck. East of 357th Avenue, it intersects SD 45. This intersection is also the western terminus of CR 2-1 (152nd Street). The two highways travel concurrently to the south. They cross over the western part of Lake Cresbard. At an intersection with 154th Street, they split, with SD 20 heading to the east. Between 360th and 361st avenues, it crosses the North Fork Snake Creek. Then, it travels just south of Cresbard. Just west of 374th Avenue, the highway crosses over Perry Creek. Just west of 372nd Avenue, it travels just south of Chelsea. At an intersection with 373rd Avenue, it enters the northwestern part of Spink County.

Spink County