B21 may refer to:

• A wine and spirits retailer in Florida
• B21 (band), a bhangra group
• B-21 (machine), a cypher machine (see Boris Hagelin)
• B21 (New York City bus) serving Brooklyn
• HLA-B21, an HLA - B serotype
• 2D line beam element in Abaqus

and also :

• North American XB-21, an experimental bomber aircraft
• Volvo Redblock Engine, an automotive engine
• Bundesstraße 21, a German road