The metal‐semiconductor interfacial photocarrier extraction and utilization are crucial for boosting photocatalytic activity, and the construction of interfacial chemical bonds to anchor the metal active sites is an emerging direction to facilitate interfacial photocarrier extraction. Herein, interfacial PtS bonds are designed via an in situ photoreduction to precisely anchor Pt sub‐nanoclusters (SNCs) on the tandem homojunction CdS (Pt‐CdSx‐T). The optimized Pt0.02‐CdSx‐T photocatalyst exhibits an impressive hydrogen evolution rate of 854.2 µmol h−1 with an apparent quantum yield of 43.55% at 470 nm, which is nearly 13 times that of pristine CdS. Systematic investigations reveal that the abundant coordination‐unsaturated S atoms on CdSx‐T act as sites to anchor Pt SNCs and the thus formed interfacial PtS bonds accelerate the metal‐semiconductor interface directional photocarriers transfer. The homojunction CdSx‐T with its tandem potential barrier promotes the separation of photocarriers via an internal electrostatic field, and the metal‐semiconductor interface electron transfer kinetics and H* adsorption capacity depend on the Pt SNCs size as confirmed by the surface photovoltage spectroscopy and density functional theory calculations. This work affords a new perspective for the exploitation of precisely dispersed cocatalysts on homojunction catalysts to achieve high‐efficiency photocatalytic hydrogen evolution.

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