40th Regiment
- 40 (Ulster) Signal Regiment
- 40th (2nd Somersetshire) Regiment of Foot
- 40th (The King's) Royal Tank Regiment
- 40th Armor Regiment
- 40th Battalion (disambiguation)
- 40th Brigade (disambiguation)
- 40th Cavalry Regiment (United States)
- 40th Division (disambiguation)
- 40th Fighter Aviation Regiment
- 40th Illinois Volunteer Infantry Regiment
- 40th Infantry Regiment (United States)
- 40th Iowa Volunteer Infantry Regiment
- 40th New York Volunteer Infantry Regiment
- 40th Regiment Royal Artillery
- 40th Squadron (disambiguation)
- 40th Wisconsin Volunteer Infantry Regiment
- 40th/41st Royal Tank Regiment