D21 may refer to:

Vehicles

Aircraft

  • Dewoitine D.21, a French fighter
  • Fokker D.XXI, a Dutch fighter
  • Lockheed D-21, an American reconnaissance drone
  • Salmson D21 Phrygane, a French utility aircraft

Ships

  • , a Nueva Esparta-class destroyer of the Venezuelan Navy
  • , a County-class destroyer of the Royal Navy
  • , a Q-class destroyer of the Royal Navy
  • , a Ruler-class escort aircraft carrier of the Royal Navy
  • , a W-class destroyer of the Royal Navy
  • , a Fletcher-class destroyer of the Spanish Navy

Surface vehicles

  • Allis-Chalmers D21, an American tractor
  • Nissan D21, a Nissan Datsun pickup truck
  • LNER Class D21, a class of British steam locomotives

Other uses

  • Arriflex D-21, a digital motion picture camera
  • Chondroma
  • D21 road (Croatia)
  • D21 – Janeček method, an electoral system
  • D21, a computer from Datasaab
  • D-21 Special, a guitar made by Martin Guitar