South Dakota Highway 251 (SD 251) is a state route in the U.S. state of South Dakota.

SD 251's southern terminus is at the Nebraska border, and the northern terminus is at U.S. Route 18 (US 18) and SD 47 in Gregory.

The entire speed limit outside of Gregory is 55 MPH, and serves a virtually unpopulated area of Gregory County.

The entire route is in Gregory County.

This South Dakota road or road transport-related article is a stub.