<p>Polarons play a crucial role in determining the (photo)electrocatalytic activity of semiconductors. Traditionally, polarons are introduced ex situ and irreversibly during catalyst synthesis, but herein we present a fundamentally different approach of introducing polarons in situ in a reversible manner using the external electrode potential. We study the potential-dependent polaron formation and its impact on electrocatalysis on a prototypical TiO₂ semiconductor electrode for the acidic hydrogen evolution reaction. By combining grand canonical ensemble density functional theory calculations with (in situ spectro) electrochemical experiments, we demonstrate notable changes in TiO₂´s electronic structure driven by the reduction of Ti⁴⁺ to Ti³⁺ surface polarons at reducing potentials. Our results show that potential-dependent polaron formation creates highly active sites for the hydrogen evolution reaction, breaks down the linear relationship between adsorption energy and electrode potential, and leads to complex electrochemical reaction kinetics. We discuss how the in situ polaron generation can be leveraged in improving semiconductor (photo)electrodes. Overall, our findings provide compelling evidence and an atomistic understanding of potential-dependent polaron formation in semiconductor (photo)electrocatalysis.</p>