M31 may refer to:
Roadways M31 motorway, a planned but unbuilt motorway in England M-31 (Michigan highway), a former state highway in Michigan Hume Highway in Australia, designated "M31" M31 motorway (Hungary), a short motorway section near Budapest, Hungary M31 (Cape Town), a Metropolitan Route in Cape Town, South Africa M31 (Johannesburg), a Metropolitan Route in Johannesburg, South Africa M31 (Pretoria), a Metropolitan Route in Pretoria, South Africa M31 (Durban), a Metropolitan Route in Durban, South Africa
Military Suomi M-31 SMG, a Finnish WWII-era submachine gun M31 HEAT rifle grenade M31 Recovery Vehicle based on the M3 Lee tank M31 rocket, an alternate designation for the MGR-1 Honest John, a United States military rocket M31 (missile), for a multiple-launch rocket system (MLRS) M31 is the pennant number for HMS Cattistock, a Hunt-class mine countermeasures vessel
Other M31 = 0x7FFFFFFF = 2147483647, the eighth Mersenne prime number M31 (New York City bus), a New York City Bus route in Manhattan M31: A Family Romance, a novel by writer Stephen Wright Messier 31 (M31), a spiral galaxy also called the Andromeda Galaxy Samsung Galaxy M31, an Android-based smartphone Remington Model 31, a pump-action shotgun made by Remington Arms