31st Regiment

• 31 (City of London) Signal Regiment
• 31 Combat Engineer Regiment (The Elgins)
• 31st (Huntingdonshire) Regiment of Foot
• 31st Battalion (disambiguation)
• 31st Brigade (disambiguation)
• 31st Cavalry Regiment (United States)
• 31st Division (disambiguation)
• 31st Group (disambiguation)
• 31st Illinois Volunteer Infantry Regiment
• 31st Infantry Regiment (Thailand)
• 31st Infantry Regiment (United States)
• 31st Infantry Regiment, United States Colored Troops
• 31st Iowa Volunteer Infantry Regiment
• 31st Mechanized Infantry Regiment "Asturias"
• 31st New York Volunteer Infantry Regiment
• 31st Reserve Field Artillery Regiment (Ireland)
• 31st Squadron (disambiguation)
• 31st Wisconsin Volunteer Infantry Regiment