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.

Major intersections

County	Location	mi	km	Destinations	Notes
Preble	Jefferson Township	0.00	0.00	SR 121 – Richmond	Southern terminus at the state line
	New Paris	1.36	2.19	SR 320	Northern terminus of SR 320
Darke	Harrison Township	7.71	12.41	SR 722	Western terminus of SR 722
	New Madison	11.21	18.04	SR 726	Northern terminus of SR 726
	Wayne Lakes	15.91	25.60	SR 503
	Neave Township	18.62	29.97	US 36
	Greenville	21.47	34.55	SR 49	Southern end of SR 49 concurrency
		22.46	36.15	SR 49 / SR 502	Northern end of SR 49 concurrency; Eastern terminus of SR 502
	Greenville Township	24.81– 24.84	39.93– 39.98	US 127	Access via temporary ramp
	Versailles	35.51	57.15	SR 185
		35.94	57.84	SR 47 / SR 185	Northern terminus
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus