Solar energy is the most abundant energy source available to humankind; however, this energy cannot be harnessed on demand due to the variability of sunlight. Artificial photosynthesis offers a sustainable way to overcome that variability through the photocatalytic conversion of solar power into chemical fuels at a semiconductor–electrolyte interface. Although considerable progress has been made in simulating the bulk properties of semiconductors from first principles, much less has been done to address the electrochemical response of electrified semiconductor–electrolyte interfaces under realistic environmental conditions. In particular, band bending and surface states are two important contributors to the photogeneration of electrical current at semiconductor–electrolyte interface. We present a broadly applicable and highly transferable computational techniques to simulate semiconductor–electrolyte interfaces. By introducing a continuum model of the semiconductor and electrolyte regions surrounding the quantum-mechanical interfacial layer, the model enables for the self-consistent determination of the surface states and of the Schottky barriers that subsequently form within the space charge of the semiconductor electrodes. Validation of these simulations is provided by the analysis of experimental charge-voltage measurements, focusing on the photovoltage region close to the onset of the photocurrent, which depends strongly on the properties of surface states. Our model provides a detailed understanding of the electrochemical stability and intrinsic current–voltage response of semiconductor electrodes taking into account the surface states that arise from the adsorption of oxygen and hydrogen species towards optimizing photocatalytic performance.