L21 may refer to:
Vehicles Aircraft Albatros L 21, a German biplane bomber Daimler L21, a German sports aircraft LZ 61 (L 21), an airship of the Imperial German Navy Piper L-21 Super Cub, an American aircraft
Ships , a submarine of the Royal Navy , a destroyer of the Royal Navy , a landing ship of the Indian Navy
Other uses 60S ribosomal protein L21 Kete language Ribosomal protein L21 leader USA-193, an American satellite