21st Regiment

21st Regiment or 21st Infantry Regiment may refer to:


 * 21 Regiment Special Air Service, a unit 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 Lancers, a unit of the British Army
 * 21st Punjab Infantry, 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 Regiment Kansas Militia Infantry
 * 21st Regiment Kentucky Volunteer Infantry
 * 21st Regiment Massachusetts Volunteer Infantry
 * 21st Wisconsin Volunteer Infantry Regiment