A31 or A-31 may refer to:

• A31 cefiro, a series of cars by Nissan
• A-31 Vengeance, an American dive bomber from the World War II era
• Archambault A31, a French sailboat design
• HLA-A31, a human serotype
• Samsung Galaxy A31, a mid-range Android smartphone
• IBM ThinkPad A31 laptop computer, produced circa 2002-2004

• Highway 31 aka Autoroute 31
• A31 motorway (Canada), a road in Quebec connecting Joliette and the A40
• A31 road (England), a road connecting Guildford, Surrey and Bere Regis, Dorset
• A31 motorway (France), a road connecting the Franco-Luxembourg border and Beaune
• A31 motorway (Germany), a road connecting the coast of the North Sea near Emden to the Ruhr area
• A31 road (Isle of Man), a road connecting Port St Mary with the Calf Sound
• A31 motorway (Italy), a road connecting Piovene Rocchette and Albettone - Barbarano Vicentino
• A31 motorway (Netherlands), a road connecting Harlingen and Leeuwarden
• A31 motorway (Spain), a road connecting Madrid and Alicante
• A31 highway (Sri Lanka), a road connecting Karaithivu and Ampara

• A code for the English Opening in the Encyclopaedia of Chess Openings
• List of highways numbered 31

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