X21 (disambiguation)
# X21 (disambiguation)

X21 may refer to:

* Northrop X-21, an experimental aircraft
* SA-X-21 or S-400 (missile), a Russian anti-aircraft weapon system
* X.21 (sometimes referred to as X21), an interface specification for differential communications
* X21 (band) (次世代ユニット), a Japanese girl idol group
