Display omitted •The feasibility of 2D metallic B2C was studied for Li/Na ion batteries.•The B2C possesses an extremely high capacity for Li/Na ions storage.•The B2C exhibits low average open circuit voltages and fast Li/Na ions diffusion.•The Li/Na-B2C systems maintain the metallic properties during adsorption. Two-dimensional (2D) materials have garnered extensive enthusiasm and become the promising candidates as electrode materials for rechargeable ion batteries owning to their high surface area and excellent electrical, thermal and mechanical properties. Recently, a new stable 2D material named B2C was theoretically proposed and possesses metallic properties. Due to its light weight and intrinsic advantage of transportation of electrons, we further assess the feasibility of B2C monolayer to serve as Li and Na ion batteries (LIBs/NIBs) anode material by using first-principles calculations. The results reveal that B2C exhibits a very high theoretical capacity of 1596 mAh·g−1 for both LIBs and NIBs. Impressively, the structure of B2C can be well maintained at the maximum concentration (Li2B2C/Na2B2C) at 400 K, suggesting its good thermal stability. Moreover, B2C offers moderate average open circuit voltages (Li: 0.57 V, Na: 0.43 V) and fast motilities with lower diffusion barriers (Li: 0.49 eV, Na: 0.23 eV). Finally, B2C can maintain metallic properties during the whole Li/Na ion insertion processes, meaning that the material has a favorable electronic conductivity. Our findings demonstrate that metallic B2C could be a potential LIBs/NIBs anode material with high performance.