A51 may refer to:
- Area 51, the nickname for a military base in Nevada that is the subject of many conspiracy theories
- A51 Terrain Park (Colorado), a terrain park in Keystone, Colorado
- A51 road (England), a road connecting Kingsbury and Chester
- A51 motorway (France), a road connecting Marseille and Grenoble
- A5/1, in cryptography, a stream cipher used in GSM cellular networks
- Samsung Galaxy A51, a smartphone released in 2019
- A51, one of the Encyclopaedia of Chess Openings codes for the Budapest Gambit in chess
- A-51, a Namibian hip hop band
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