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 , 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