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 aircraft 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