21st Regiment

• 21 Engineer Regiment (United Kingdom)
• 21 Squadron (disambiguation)
• 21st Brigade (disambiguation)
• 21st Construction Regiment (Australia)
• 21st Continental Regiment
• 21st Division (disambiguation)
• 21st Foot
• 21st Illinois Volunteer Infantry Regiment
• 21st Infantry Regiment (Thailand)
• 21st Infantry Regiment (United States)
• 21st Iowa Volunteer Infantry Regiment
• 21st Kansas Militia Infantry Regiment
• 21st Lancers
• 21st Maine Volunteer Infantry Regiment
• 21st Marine Infantry Regiment
• 21st Marine Regiment (United States)
• 21st Michigan Volunteer Infantry Regiment
• 21st Mississippi Infantry Regiment
• 21st Ohio Infantry
• 21st Punjabis
• 21st Regiment Kentucky Volunteer Infantry
• 21st Regiment Massachusetts Volunteer Infantry
• 21st Wisconsin Volunteer Infantry Regiment
• Artists Rifles
• Regiment
• XXI Corps (disambiguation)