South Dakota Highway 18

Standard South Dakota highway shields
System information
Notes:	South Dakota highways are generally state-maintained.
Highway names
Interstates:	Interstate X (I-X)
US Highways:	U.S. Highway X (US X)
State:	(State) Highway X (SD X)

The following is a list of former state highways in South Dakota. These roads are now either parts of other routes or no longer carry a route number.

South Dakota Highway 8 was one of only two single digit state highway numbers known to be used in South Dakota. It was a state route across north central and northwest South Dakota, generally following what is now South Dakota Highway 20. The first designation of this route, in 1926, was South Dakota Highway 18. By the late 1920s, U.S. Highway 18 was established across southern South Dakota. The existence of two highway 18's was corrected around 1935, when the northern highway was redesignated as South Dakota 8. This number remained in use until the late 1960s, when SD 20 was extended west across the Missouri River, absorbing the SD 8 alignment.

South Dakota Highway 9 was a designation that was used twice.

This road went from Minnesota west to Sioux Falls and then southeast to Iowa. When the U.S. highway system was implemented in 1926, the north half of road was designated as part of U.S. Highway 16. After a short period of dual signage, the SD 9 designation was dropped, as the rest had been transferred to SD 38.