D21

D21 may refer to:

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

 * ARV Zulia, a Nueva Esparta-class destroyer of the Venezuelan Navy
 * HMS Norfolk (D21), a County-class destroyer of the Royal Navy
 * HMS Quickmatch (D21), a Q-class destroyer of the Royal Navy
 * HMS Shah (D21), a Ruler-class escort aircraft carrier of the Royal Navy
 * HMS Wryneck (D21), a W-class destroyer of the Royal Navy
 * SPANISH DESTROYER Lepanto, a Fletcher-class destroyer of the Spanish Navy

Surface vehicles

 * Allis-Chalmers D21, an American tractor
 * D21 Navara, 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