C31 or C-31 may be: * C-31 highway (Spain), a primary highway in Catalonia * HMS C31, a 1909 British C class submarine * , an American PRR 0-8-0 wheel arrangement steam locomotive * Caldwell 31 (IC 405, the Flaming Star Nebula), an emission/reflection nebula in the constellation Auriga * an identifier given to the Blackstripe corydoras (Corydoras bondi), a freshwater catfish, prior to its identification * a molecule important in diapedesis * a molecular fragment of amyloid precursor protein that is believed to upregulate mechanisms of apoptosis (Zheng and Koo, Molecular Neuroegeneration 2006, 1:5, DOI: 10.1186/1750-1326-1-5) * Channel 31 as in C31 Adelaide and C31 Melbourne, two Australian community television stations * King's Gambit, an Encyclopaedia of Chess Openings code * Ma