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 ==