A multiphysics model was developed for a photoelectrochemical (PEC) cell at the device level to simulate water splitting operating under concentrated irradiation (between 50 to 600 kW m−2). The 2D model couples charge, heat, mass, photon, and momentum transfer to predict local current densities, potential distributions, temperature profiles, volumetric gas fractions, pressure, and velocities profiles in the electrolyte. Electrode kinetics and electrolyte resistance were considered for the electrochemical processes, and two-phase bubbly- flow under laminar conditions for momentum transfer. The effects of bubbles on the incident photon flux and the thermal and electrical conductivities of the electrolyte were also considered. Photocurrent densities were estimated using a semiempirical correlations dependent on potential, charge transfer efficiencies, and temperature. The model was applied to a custom-made cell utilising a spray pyrolysed Sn-doped Fe2O3 photoanode, a material with well-known photoelectrochemical behaviour and stability. Transparent conductive glass and titanium foil were investigated as two possible photoanode substrates. Predictions indicate that commercial conductive glasses are not suitable substrates due to a significant ohmic drop caused by high current densities. The model illustrates that thermal and bubble management are critical to improving the overall performance of a PEC cell subjected to high photon flux. Furthermore, the model can be used to decouple the phenomena that occur under such conditions and could assist in the study of photoelectrode materials under high irradiance.