Type 21 can refer to:
Type 21 frigate, or Amazon-class frigate, a late-20th-century escort vessel of the Royal Navy. Type 021-class missile boat, a Chinese missile-firing naval vessel first produced in about 1975, and still in production. Type XXI submarine, a German technologically advanced submarine produced in 1945. Type 21 Radar, a radar of the Imperial Japanese Navy
See also Class 21 (disambiguation)