91st Regiment
- 91st Cavalry Regiment
- 91st Coast Artillery (United States)
- 91st Division (disambiguation)
- 91st Illinois Volunteer Infantry Regiment
- 91st Indiana Infantry Regiment
- 91st New York Volunteer Infantry Regiment
- 91st Ohio Infantry
- 91st Pennsylvania Infantry
- 91st Punjabis (Light Infantry)
- 91st Regiment of Foot (disambiguation)