121st Regiment
- 121st Anti-aircraft Artillery Regiment "Ravenna"
- 121st Battalion (disambiguation)
- 121st Brigade (disambiguation)
- 121st Division (disambiguation)
- 121st Field Artillery Regiment (United States)
- 121st Infantry Regiment "Macerata"
- 121st Infantry Regiment (France)
- 121st Infantry Regiment (United States)
- 121st New York Infantry Regiment
- 121st Ohio Infantry Regiment
- 121st Pennsylvania Infantry Regiment
- 121st Regiment (Xinjiang Production and Construction Corps)
- 121st Regiment of Foot (disambiguation)