South Dakota Highway 109 (SD 109) is a 12.167-mile (19.581 km) state highway in Grant and Roberts counties in South Dakota, United States, runs along the south side of Big Stone Lake and the Minnesota border from U.S. Route 12 in Big Stone City to South Dakota Highway 15.

SD 109 begins at an intersection with US 12 in Big Stone City.

It immediately crosses a bridge spanning railroad tracks and it heads northwest out of town.

The highway follows the curvature of Big Stone Lake on the Minnesota River to the northwest and west for 12 miles (19 km).

It ends at a 90-degree bend in SD 15 east of Hartford Beach State Park.