In military terms, 21st Brigade may refer to:
Australia 21st Brigade (Australia)
Germany 21st Panzer Brigade (Bundeswehr) 21st Panzer Brigade (Wehrmacht)
Greece 21st Armoured Brigade (Greece) 21st Brigade (Greece), an infantry brigade
India 21st Indian Infantry Brigade 21st (Bareilly) Brigade
Indonesia 21st Infantry Brigade (Indonesia)
Japan 21st Independent Mixed Brigade (Imperial Japanese Army)
Namibia 21 Motorised Infantry Brigade (Namibia)
Poland Polish 21st Podhale Rifles Brigade
Russia 21st Separate Guards Motor Rifle Brigade
Soviet Union 21st Air Assault Brigade (Soviet Union) 21st Missile Brigade
Ukraine 21st Mechanized Brigade (Ukraine) 21st Public Order Brigade (Ukraine)
United Kingdom 21st Army Tank Brigade (United Kingdom) 21st Brigade (United Kingdom) 21st (East Africa) Infantry Brigade 21st Mounted Brigade 21st Reserve Brigade 21st Brigade, Royal Field Artillery
United States 21st Cavalry Brigade 21st Signal Brigade
See also 21st Division (disambiguation) 21st Regiment (disambiguation) 21 Squadron (disambiguation)