A51

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