J21

J21 or J-21 may refer to:

Aircraft

 * Junkers J 21, a German reconnaissance aircraft
 * SAAB J 21, a Swedish fighter
 * Shenyang J-21, a Chinese prototype fighter
 * Soko J-21 Jastreb, a Yugoslav light attack aircraft

Locomotives

 * LNER Class J21, a British steam locomotive class

Ships

 * HMCS Canso (J21), a Bangor-class minesweeper of the Royal Navy and Royal Canadian Navy
 * HSWMS Södermanland, an Östergötland-class destroyer of the Swedish Navy
 * INS Darshak, a survey ship of the Indian Navy

Other uses

 * Bronchiolitis
 * County Route J21 (California)
 * Elongated pentagonal rotunda, a Johnson solid (J21)