N21 or N-21 may refer to:
Buses and trams
- N21 (Long Island bus), New York
- London Buses route N21
Roads
- N21 road (Belgium), a National Road in Belgium
- Route nationale 21, in France
- N21 road (Ireland)
- N21 (Cape Town), in South Africa
- Nebraska Highway 21, in the United States
Other uses
- HMS Abdiel (N21), a minelayer of the Royal Navy
- Network TwentyOne, an Amway support organization
- Nieuport 21, a French First World War fighter
- Nitrogen-21, an isotope of nitrogen
- Tumbuka language
- N21, a postcode district in the N postcode area
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