321st
- 1st Battalion, 321st Field Artillery Regiment
- 2nd Battalion, 321st Field Artillery Regiment
- 321
- 321 (number)
- 321 BC
- 321st Air Expeditionary Group
- 321st Air Expeditionary Wing
- 321st Air Refueling Squadron
- 321st Division (Imperial Japanese Army)
- 321st Engineer Battalion (United States)
- 321st Field Artillery Regiment
- 321st Fighter-Interceptor Squadron
- 321st Missile Squadron
- 321st Missile Wing LGM-30 Minuteman Missile Launch Sites
- 321st Rifle Division (Soviet Union)
- 321st Special Tactics Squadron
- 321st Sustainment Brigade (United States)
- 3rd Battalion, 321st Field Artillery Regiment