In this work, we demonstrate an effective surface modulation strategy via facet-selective photodeposition coupled with oxygen vacancy-mediated size control of palladium (Pd) cocatalysts on monoclinic BiVO4 for efficient visible-light-driven H2O2 synthesis. By adjusting the pH during synthesis, a series of monoclinic BiVO4 with different degrees of oxygen vacancies is obtained; higher pH induces elevated vacancy levels, whereas only low to moderate pH preserves the decahedral structure with distinguished facets. Upon facet-selective photodeposition of Pd metal onto the (010) surface, increased oxygen vacancy concentrations enhance the electrostatic interactions with PdCl42− precursors, leading to the formation of smaller Pd nanoparticles with higher active-site density and accelerated surface reaction kinetics. However, the loss of well-defined facet geometry of the BiVO4 prepared under high pH weakens intrinsic spatial charge separation and compromises facet-selective Pd photodeposition, eventually diminishing photocatalytic activity. Thus, the optimized Pd/BiVO4 photocatalyst achieves an excellent H2O2 production rate of 2.35 mм/h, with an apparent quantum efficiency (AQE) of 19.2% at 420 nm, representing a significant performance advancement among BiVO4-based systems. This study not only sets a new benchmark but also offers valuable insights into the rational integration of defect and facet engineering for designing efficient metal/semiconductor composites for photocatalytic H2O2 production. © 2025 Wiley-VCH GmbH.