We have combined our state-of-the-art constant potential methodology with various electrocatalytic experiments to show that the presence of electronic polarons and defects activate TiO2 towards HER. Both computational and experimental results demonstrate distinct changes in the electronic structure of TiO2 as a function of electrode potential, resulting in the reduction Ti 4+ atoms to Ti 3+ polarons at the TiO2 surface. They in turn control the HER thermodynamics, kinetics, conductivity, and the overall HER activity along the Volmer--Heyrovsky mechanism. As the polaron formation results in significant and non-linear changes in the surface charge and reaction energetics as a function of the electrode potential and defect concentration, understanding and optimizing semiconductor electrocatalysts needs complementary insight from experiments and calculations. In addition to offering fundamental insight into HER on semiconducting electrodes, we suggest that leveraging the synergy between defects and potential-dependent polaron formation could serve as a strategy to design improved (photo)electrocatalysts.