Mk 21

Mark 21 or Mk XXI or variation, may refer to:


 * 5"/38 MK 30, a variant of the 5-inch/38-caliber gun
 * Beaufighter Mark 21, a variant of the Bristol Beaufighter
 * Mk 21 (type 356), a variant of the Supermarine Spitfire (Griffon-powered variants)
 * Freighter Mk 21, a variant of the Bristol Freighter
 * Mk 21 Mod 0, a 7.62×51mm variant of the M1919 Browning machine gun
 * Mk. 21 Airborne Interception radar
 * Mark 21 nuclear bomb
 * Mk 21 Precision Sniper Rifle (PSR), based on the Remington MSR
 * MK 21 Mod 0 Scalable Offensive Hand Grenade