81st Regiment
- 81st Assault Aviation Regiment
- 81st Brigade (disambiguation)
- 81st Division (disambiguation)
- 81st Illinois Volunteer Infantry Regiment
- 81st Indiana Infantry Regiment
- 81st Infantry Regiment (Imperial Japanese Army)
- 81st New York Volunteer Infantry
- 81st Ohio Infantry
- 81st Pennsylvania Infantry
- 81st Regiment of Foot (Aberdeenshire Highland Regiment)
- 81st Regiment of Foot (Loyal Lincoln Volunteers)
- 81st Squadron (disambiguation)
- Philippine Commonwealth Army