Abstract: The utilization of renewable solar energy to convert CO₂ into high-value-added chemicals and fuels is of great significance for mitigating the greenhouse effect and achieving carbon neutrality. Although ZnIn₂S₄ (ZIS) is a promising photocatalyst for CO₂ reduction, its practical application is severely limited by rapid photogenerated carrier recombination and photocorrosion. Constructing heterojunctions with g-C₃N₄ (CN) is an effective strategy; however, conventional methods often result in poor interfacial contact and involve complex synthesis processes. In this study, an electrostatic self-assembly strategy was employed to combine negatively charged ultrathin ZIS nanosheets with protonated, positively charged g-C₃N₄ (pCN) at pH 3, forming a ZnIn₂S₄/pCN (ZIS/pCN) heterojunction with a 2D/2D face-to-face configuration and enabling controllable interface engineering. Without sacrificial agents or noble-metal cocatalysts, the optimal ZIS/pCN (5:3) composite exhibited significantly enhanced photocatalytic activity for CO₂ photoreduction in a gas-solid reaction system, achieving CO and CH₄ evolution rates of 14.72 and 1.42 μmol·g⁻¹·h⁻¹, respectively—10.2 and 7.5 times higher than those of pure ZIS—while maintaining excellent photostability. In addition, ZIS/pCN (5:3) exhibited the highest CH₄ selectivity, indicating that the optimized 2D/2D heterojunction kinetically favors a multi-electron transfer pathway. This composite effectively addresses the intrinsic limitations of single-component materials, including severe charge recombination and active-site masking in ZIS, as well as the poor interfacial contact in conventional heterojunctions. Optical characterization, X-ray photoelectron spectroscopy (XPS), and photoelectrochemical measurements demonstrate that the heterojunction broadens visible-light absorption and establishes an internal electric field via interfacial electron interactions. Upon light irradiation, both pCN and ZIS are excited to generate electron-hole pairs. Driven by the internal electric field, charge carriers follow an S-scheme transfer pathway across the interface, in which high-energy electrons retained in the conduction band of pCN reduce CO₂ to CO and CH₄, while holes in the valence band of ZIS participate in water oxidation to supply protons. The 2D/2D face-to-face configuration provides a large and intimate contact interface, serving as a continuous pathway for charge transfer and significantly reducing the transport resistance. Consequently, this unique heterostructure markedly enhances charge separation efficiency and reaction kinetics. This work provides a reliable interface engineering strategy for designing efficient and stable photocatalysts for CO₂ reduction, highlighting the synergistic effect of precisely constructed heterojunctions in boosting photocatalytic performance.