From Mobridge to the Walworth–Potter county line, this highway is part of the Lewis and Clark Trail.

From about Timber Lake to just west of Mobridge, this highway is part of the Native American Scenic Byway.

The much-shorter eastern segment extends 39 miles (63 km) 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 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.

Almost immediately after entering Camp Crook, the highway's dirt portion ends.

It then intersects the northern terminus of South Camp Crook Road, which leads to Sky Ranch and Belle Fourche.

It intersects the northern terminus of Park Road, which leads to the Antelope Range and Livestock Research Station.

A little ways east of 200th Avenue, it enters the northeast part of both Ziebach County and the Cheyenne River Indian Reservation.

They skirt along the southern edge of Isabel Municipal Airport and then curve to the north.

It crosses the Little Moreau River, and almost immediately, intersects the southern terminus of County Road 243 (243rd Avenue) and SD 63.

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.

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 Indian Reservation and enter the northwestern part of Walworth County.

US 12 and SD 20 cross over some railroad tracks of BNSF Railway and curve to the south.

Just south of Walworth Street, they cross over some railroad tracks of BNSF Railway and pass some sewage disposal ponds.

At an intersection with the eastern terminus of CR 8 (157th Street), the two highways split, with SD 20 continuing its eastward trek.

At an intersection with 436th Avenue, which is northwest of Wallace, it enters the northwestern part of Codington County.

East of 441st Avenue, and north of Grass Lake, the highway curves to the southeast.

Between 161st Street and 445th Avenue, it skirts along the eastern edge of Dry Lake State Public Shooting Area.

After it begins a curve to the south, it travels through the western part of the American Game Association State Public Shooting Area.

South of 163rd Street, it curves to the southeast and travels just northeast of the Chain of Lakes.

It curves to the east-southeast and intersects North Lake Drive, which was the northern terminus of SD 139.

An intersection with Kemp Avenue West leads to the business district, the Watertown Stadium, and the Codington County Extension Center and Agricultural Building.

East of 462nd Avenue, it curves to the southeast and crosses over some railroad tracks of BNSF Railway.

Just south of 161st Street, which is east-southeast of La Bolt, the two highways cross over some railroad tracks of BNSF Railway.

The only segment of SD 20 that is included as part of the National Highway System, a system of routes determined to be the most important for the nation's economy, mobility and defense, is the portion concurrent with US 12 and US 83 from west-northwest of Mobridge to southwest of Lowry.

Also, north of Watertown, a stub was added between US 81's former path (current western terminus of the eastern segment) and what was US 77 (now SD 15).

A few years later, it was shifted to travel between US 77/SD 20, north-northeast of La Bolt, to the Minnesota state line, northeast of Albee.