In military terms, 91st Division or 91st Infantry Division may refer to:
Infantry divisions:
- 91st Infantry Division (German Empire)
- 91st Infantry Division (Wehrmacht)
- 91st Division (Israel)
- 91st Division (Imperial Japanese Army)
- 91st Division (Philippines)
- 91st Division (United States)
- 91st Rifle Division, Soviet Union
- 91st Motor Rifle Division, Soviet Union
Aviation divisions:
- 91st Air Division, United States