21st Regiment

21st Regiment may refer to:

Infantry regiments

 * 21 Regiment Special Air Service, a unit of the British Army
 * 21st Foot, a former regiment of the British Army
 * 21st Infantry Regiment (Thailand), a unit of the Royal Thai Army
 * 21st Infantry Regiment (United States), a unit of the United States Army
 * 21st Punjabis, a unit of the British Indian Army
 * 21st Marine Infantry Regiment, a unit of the French Marines
 * 21st Marine Regiment (United States), a unit of the United States Marine Corps

American Revolutionary War regiments

 * 21st Continental Regiment

American Civil War regiments

 * 21st Iowa Volunteer Infantry Regiment
 * 21st Illinois Volunteer Infantry Regiment
 * 21st Maine Volunteer Infantry Regiment
 * 21st Michigan Volunteer Infantry Regiment
 * 21st Mississippi Infantry Regiment
 * 21st Ohio Infantry
 * 21st Kansas Militia Infantry Regiment
 * 21st Regiment Kentucky Volunteer Infantry
 * 21st Regiment Massachusetts Volunteer Infantry
 * 21st Wisconsin Volunteer Infantry Regiment

Cavalry regiments

 * 21st Lancers, a unit of the British Army

Engineer regiments

 * 21st Construction Regiment (Australia), a former unit of the Australian Army's Engineers
 * 21 Engineer Regiment (United Kingdom), a unit of the British Army's Royal Engineers