X21 (disambiguation)

X21 may refer to:

• Northrop X-21, experimental aircraft designed to test wings with laminar flow control
• SA-X-21 or S-400 (missile), anti-aircraft weapon system developed by Russia's Almaz Central Design Bureau as an upgrade of the S-300 family
• X.21 (sometimes referred to as X21), an interface specification for differential communications introduced in the mid-1970s by the ITU-T
• X21 (band) (次世代ユニット), a Japanese girl idol group