[3] SD 123 runs along a two-lane section of 467th Avenue, beginning at an intersection with US 12 in Grant County, about 2.5 miles (4.0 km) east of Marvin.

US 12 heads easterly to pass north of Twin Brooks, through Milbank, and on to its national western terminus in Detroit, Michigan.

Within Wilmot, SD 123 crosses Ordway, 1st, and 2nd streets before connecting with the west ends of Main, 3rd, and 4th streets before reaching its northern terminus at a junction with SD 15 and Roberts County Road 9 (CR 9) on the northern edge of the city.

(Within Roberts County, except for the section within the city of Wilmot, SD 123 runs along the border between Springdale and Garfield civil townships.)

SD 15 heads west, along the border between Springdale and Lee townships to enter the Lake Traverse Indian Reservation and end at a junction with Robert County Road 17 and Roberts County Road 34, just west of SD 15's interchange with I‑29/US 81 at Exit 213.