| State: | SD |
| Type: | SD |
| Route: | 109 |
| Section: | 190 |
| Map Notes: | Route of SD 109 (in red) |
| Length Mi: | 12.167 |
| Length Ref: | [1] |
| Established: | 1975[2] |
| History: | SD 15Y: 1962 - 1975 |
| Direction A: | South |
| Terminus A: | in Big Stone City |
| Direction B: | North |
| Terminus B: | in Geneseo Township |
| Counties: | Grant, Roberts |
| Previous Type: | SD |
| Previous Route: | 106 |
| Next Type: | SD |
| Next Route: | 115 |
South Dakota Highway 109 (SD 109) is a 12.167miles 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 12miles. It ends at a 90-degree bend in SD 15 east of Hartford Beach State Park.
SD 109 was originally designated Highway 15Y. It was renumbered to 109 in 1975.