The oxygen evolution reaction (OER) at the BiVO4‐water interface is considered to be the bottleneck of the overall water splitting at this aqueous interface. To provide insight into the mechanism of this reaction, we focus on the first proton‐coupled electron transfer (PCET). We obtain the free‐energy surface of this first step by combining on‐the‐fly probability enhanced sampling and machine learning potentials at the hybrid functional level of accuracy. Our study reveals that proton transfer precedes electron transfer and determines the reaction barrier, consistent with kinetic‐isotope‐effect experiments. The calculated reaction barrier amounts to 0.66±0.09 eV. The proton moves from the adsorbed water molecule to a surface O atom through a direct transfer mechanism. The hole hopping to the resulting hydroxide occurs via the mediation of a surface Bi atom. The presented framework can be generally applied to other PCET steps and other oxides, thus opening the door to a comprehensive investigation of the OER mechanism at oxide‐water interfaces.