SD 123 runs along a two-lane section of 467th Avenue, beginning at an intersection with US 12 in Grant County, about east of Marvin. 467th Avenue continues south as a dirt road to connect with County Road 14 and end at County Road 26. US 12 heads westerly to pass by the north side of both Marvin and Summit, before an interchange with Interstate 29/U.S. Route 81 at Exit 207, then on to its national western terminus in Aberdeen, Washington. US 12 heads easterly to pass north of Twin Brooks, through Milbank, and on to its national western terminus in Detroit, Michigan. From its southern terminus SD 123 heads north through open plains to cross 146th Street and 145th Street. The route then crosses 144th Street just before crossing over an unnamed street and then crosses 143rd Street. The next road crossing is 142nd Street, which runs along the border between Grant and Roberts counties. Within Grant County, SD 123 runs along the border between Osceola and Kilborn civil townships.
Upon crossing 142nd Street, about north of its southern terminus, SD 123 enters Roberts County and almost immediately crosses over another unnamed stream. Continuing north through rolling farmland the route crosses 141st Street and then 140th Street. After crossing 139th Street, SD 15 crosses over one more unnamed stream before crossing 138th Street. Shortly after crossing 137th Street, the route enters the only populated place along the route, the city of Wilmot. 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 on the northern edge of the city. CR 9 continues north along 467th Avenue, on the border between Lee and Lake civil townships, to connect with Roberts County Road 24 before reaching the Minnesotastate line, on the edge of the city of Browns Valley, Minnesota. 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. SR 15 heads east to along the border between Garfield and Lake townships to Big Stone Lake, before turning south, passing through the city of Milbank, and eventually ending at an interchange with I‑29/US 81 at Exit 150.
Traffic
In 2011, the South Dakota Department of Transportation measured the amount of traffic on its highways in average annual daily traffic. SD 123 had an average of 217 vehicles, including 31 trucks. SDDOT's 2018 report does not provide data for SD 123, however, the January 2019 Aberdeen Region Highway Log reports average daily traffic of 197 vehicles for the segment south of Wilmot and 375 vehicles within Wilmot. SD 123 is not a part of the National Highway System, a system of highways important to the nation's defense, economy, and mobility.
