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Transition metal (TM) single-atom catalysts (SACs) have been widely applied in photocatalytic CO₂ reduction. In this work, <i>n</i>–<i>p</i> codoping engineering is introduced to account for the modulation of photocatalytic CO₂ reduction on a two-dimensional (2D) bismuth-oxyhalide-based cathode by using first-principles calculation. <i>n</i>–<i>p</i> codoping is established via the Coulomb interactions between the negatively charged TM SACs and the positively charged <i>Cl</i> vacancy (<i>V_Cl</i>) in the dopant–defect pairs. Based on the formation energy of charged defects, neutral dopant–defect pairs for the Fe, Co, and Ni SACs (<i>P_TM</i>⁰) and the −1<i>e</i> charge state of the Cu SAC-based pair (<i>P_Cu</i>⁻¹) are stable. The electrostatic attraction of the <i>n</i>–<i>p</i> codoping strengthens the stability and solubility of TM SACs by neutralizing the oppositely charged <i>V_Cl</i> defect and TM dopant. The <i>n</i>–<i>p</i> codoping stabilizes the electron accumulation around the TM SACs. Accumulated electrons modify the <i>d</i>-orbital alignment and shift the <i>d</i>-band center toward the Fermi level, enhancing the reducing capacity of TM SACs based on the d-band theory. Besides the electrostatic attraction of the <i>n</i>–<i>p</i> codoping, the <i>P_Cu</i>⁻¹ also accumulates additional electrons surrounding Cu SACs and forms a half-occupied <i>d_x</i>²_−<i>y</i>² state, which further upshifts the <i>d</i>-band center and improves photocatalytic CO₂ reduction. The metastability of <i>Cl</i> multivacancies limits the concentration of the <i>n</i>–<i>p</i> pairs with <i>Cl</i> multivacancies (<i>P_TM@nCl</i> (n > 1)). Positively charged centers around the <i>P_TM@nCl</i> (n > 1) hinders the CO₂ reduction by shielding the charge transfer to the CO₂ molecule.