21st Division
- 21st Air Division (United States)
- 21st Brigade (disambiguation)
- 21st Cavalry Division (United States)
- 21st Division (German Empire)
- 21st Division (Imperial Japanese Army)
- 21st Division (People's Republic of China)
- 21st Division (South Vietnam)
- 21st Division (Spain)
- 21st Division (United Kingdom)
- 21st Infantry Division (France)
- 21st Infantry Division (India)
- 21st Infantry Division (Iran)
- 21st Infantry Division (Russian Empire)
- 21st Infantry Division (Wehrmacht)
- 21st Infantry Division Granatieri di Sardegna
- 21st Mountain Infantry Division (Poland)
- 21st Panzer Division (Wehrmacht)
- 21st Regiment (disambiguation)
- 21st Reserve Division (German Empire)
- 21st Waffen Mountain Division of the SS Skanderbeg