South Dakota Highway 73 (SD 73) is a state route that runs across western South Dakota.

When designated in the 1920s, South Dakota 73 only consisted of the segment from Faith (U.S. Route 212) to Lemmon (U.S. Route 12).

Around 1935, SD 73 was extended south from Faith, absorbing what had been a portion of South Dakota Highway 24 to near Plainview, then southeast via a new route to Philip.

It continued south to the Nebraska border south of Martin, after sharing an alignment with U.S. Highway 18.

Around 1979, the northern terminus was extended east along U.S. 12 to Thunder Hawk, where it met up with a new road to link with ND 49.