21st Regiment
- 21 Engineer Regiment (United Kingdom)
- 21 Special Air Service Regiment (Artists) (Reserve)
- 21 Squadron (disambiguation)
- 21st Brigade (disambiguation)
- 21st Construction Regiment (Australia)
- 21st Continental Regiment
- 21st Division (disambiguation)
- 21st Foot
- 21st Illinois Infantry Regiment
- 21st Infantry Regiment (Thailand)
- 21st Infantry Regiment (United States)
- 21st Iowa Infantry Regiment
- 21st Kansas Militia Infantry Regiment
- 21st Kentucky Infantry Regiment
- 21st Lancers
- 21st Maine Infantry Regiment
- 21st Marine Infantry Regiment
- 21st Marine Regiment (United States)
- 21st Massachusetts Infantry Regiment
- 21st Michigan Infantry Regiment
- 21st Mississippi Infantry Regiment
- 21st Ohio Infantry Regiment
- 21st Punjabis
- 21st Wisconsin Infantry Regiment
- Regiment
- XXI Corps (disambiguation)