18th Regiment
- 18 (UKSF) Signal Regiment
- 18 Squadron (disambiguation)
- 18th Arkansas Infantry Regiment (Carroll's)
- 18th Arkansas Infantry Regiment (Marmaduke's)
- 18th Battalion (disambiguation)
- 18th Brigade (disambiguation)
- 18th Connecticut Infantry Regiment
- 18th Division (disambiguation)
- 18th Georgia Volunteer Infantry
- 18th Illinois Volunteer Infantry Regiment
- 18th Infantry (British Indian Army)
- 18th Infantry Regiment (Imperial Japanese Army)
- 18th Infantry Regiment (United States)
- 18th Iowa Volunteer Infantry Regiment
- 18th Kansas Militia Infantry Regiment
- 18th Marine Regiment (United States)
- 18th Michigan Volunteer Infantry Regiment
- 18th Missouri Volunteer Infantry
- 18th New Hampshire Volunteer Infantry
- 18th New York Volunteer Infantry
- 18th North Carolina Infantry
- 18th Ohio Infantry
- 18th Regiment "Edolo"
- 18th Regiment Alabama Infantry
- 18th Regiment Indiana Infantry
- 18th Regiment Kentucky Volunteer Infantry
- 18th Regiment Massachusetts Volunteer Infantry
- 18th Royal Hussars
- 18th U.S. Colored Infantry
- 18th Vermont Volunteer Infantry Regiment
- 18th Virginia Cavalry
- 18th Virginia Infantry
- 18th Wisconsin Volunteer Infantry Regiment
- Royal Irish Regiment (1684–1922)
- XVIII Corps (disambiguation)