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Abstract Determining suitable dopants with optimized doping concentration is critical to design efficient water splitting photocatalysts. However, there is currently a lack of fundamental knowledge to guide this process. Herein, we examine the impact of Al³⁺, Mg²⁺, and Ga³⁺on the photocatalytic performance of SrTiO₃and propose a defect compensation model to understand the doping effect. Doped SrTiO₃crystals were grown hydrothermally and treated in molten SrCl₂. The hydrogen production rates from 50 catalysts produced in this way were measured with a high‐throughput parallelized and automated photochemical reactor (PAPCR). The investigation revealed that all three dopants significantly enhance the photocatalytic reactivity. According to Brouwer diagrams computed using available reaction constants, the optimum reactivity is achieved when the concentration of acceptor dopants fully compensates the oxygen vacancy donors. The improved reactivity can be attributed to the reduction in free electron concentration, resulting in a space charge layer that is 1000 times longer. Consequently, this situation enhances the number of photogenerated charge carriers capable of being separated by the band bending and transported to the surface.