| State: | OH |
| Type: | SR |
| Route: | 121 |
| Map: | OH 121 map.svg |
| Length Mi: | 35.94 |
| Length Round: | 2 |
| Established: | 1924 |
| Direction A: | South |
| Terminus A: | near New Paris |
| Junction: |
|
| Direction B: | North |
| Terminus B: | in Versailles |
| Counties: | Preble, Darke |
| Previous Type: | SR |
| Previous Route: | 120 |
| Next Type: | SR |
| Next Route: | 122 |
State Route 121 (SR 121) is a state highway in western Ohio. It starts at Indiana State Road 121 near New Paris, Ohio, and ends at State Route 47 and State Route 185 in Versailles, Ohio.