Presently, great attention is being directed toward the development of promising electrode materials for non-lithium rechargeable batteries which have the advantages of low cost, high energy storage density, and high rate capacity for substantial renewable energy applications. In this study, we have predicted that the C3N monolayer is a potential electrode material for Na- and K-ion batteries by first-principle calculations. The diffusion barriers are calculated to be as small as 0.03 eV for Na and 0.07 eV for K, which could lead to a very fast diffusion on the C3N monolayer surface, implying high mobility and cycle stability for batteries. The C3N monolayer is predicted to allow a high storage capacity of 1072 mAh/g by the inclusion of multilayer adsorption with an average voltage of 0.13 V for Na2C3N and 0.26 V for K2C3N systems, which is more promising than previously studied anode materials. All of these results ensure that the C3N monolayer could serve as an excellent anode material for Na- and K-ion batteries. © 2018 American Chemical Society.