X21 may refer to:

• X.21, a telecommunications standard
• X21 (group), a Japanese girl idol group
• X21 (New York City bus)
• Northrop X-21, an experimental aircraft
• SA-X-21, a Russian anti-aircraft weapon system
• TGOJ X21, a Swedish electric multiple unit
• ThinkPad X21, a notebook computer
• Banham X21, a kit car from Banham Conversions