Class 21

Class 21 or 21 Class may refer to:
 * Atlantic 21-class lifeboat
 * Belgian Railways Class 21, Belgian electric locomotive
 * British Rail Class 21 (NBL), British diesel-electric locomotive
 * British Rail Class 21 (MaK), various locomotives used by Eurotunnel
 * GS&WR Class 21
 * Hormuz 21-class landing ship
 * L&YR Class 21, British 0-4-0ST steam locomotive
 * NSB Class 21, Norwegian 2-6-0 steam locomotive
 * O 21-class submarine
 * P21-class patrol vessel
 * PKP class ET21
 * South African Class 21 2-10-4, South African steam locomotive