The design of efficient and stable photocatalysts for robust CO₂ reduction without sacrifice reagent or extra photosensitizer is still challenging. Herein, a single-atom catalyst of isolated single atom cobalt incorporated into Bi₃O₄Br atomic layers is successfully prepared. The cobalt single atoms in the Bi₃O₄Br favors the charge transition, carrier separation, CO₂ adsorption and activation. It can lower the CO₂ activation energy barrier through stabilizing the COOH* intermediates and tune the rate-limiting step from the formation of adsorbed intermediate COOH* to be CO* desorption. Taking advantage of cobalt single atoms and two-dimensional ultrathin Bi₃O₄Br atomic layers, the optimized catalyst can perform light-driven CO₂ reduction with a selective CO formation rate of 107.1 µmol g⁻¹ h⁻¹, roughly 4 and 32 times higher than that of atomic layer Bi₃O₄Br and bulk Bi₃O₄Br, respectively.