Using DFT calculations, a novel metal-free catalyst is proposed for reduction of nitrous oxide (N2O). It is found that the presence of a B impurity can modify the surface reactivity of C3N and promote the effective adsorption of N2O. The reduction of N2O over B-doped C3N nanosheets starts with the adsorption of this molecule, followed by its decomposition into N-2 and an active oxygen atom (O*). By overcoming a small activation energy, the O* moiety can be then eliminated by a CO molecule. The results suggest that B-doped C3N nanosheets are promising catalysts to remove the environmentally harmful N2O molecules.