Ferroelectric modulation of the surface charge density is a promising strategy to promote the surface oxygen evolution reaction (OER) in photocatalytic water splitting. The limitations of the Sabatier principle could be overcome by tuning the interaction strength between the OER intermediates and the surface for the individual reaction steps via switching of the polarization direction. InSnO 2 N, a newly reported improper ferroelectric semiconductor, is promising for applications as a photocatalyst due to its direct band gap of 1.61 eV and its sizable ferroelectric polarization. Therefore, in this work, we use density functional theory to investigate the OER performance on its (001) surface as a function of the bulk polarization direction. We find that the clean surface of the downward (negatively) polarized bulk structure (“polarized bulk” henceforth) has a lower theoretical overpotential of η = 0.58 V versus the standard hydrogen electrode compared to the clean surface with an upward (positively) polarized bulk structure (0.77 V). Under (photo)-electrochemical operating conditions, a monolayer (ML) OH-covered surface is the most stable for the negatively polarized bulk and shows a theoretical overpotential of 0.89 V, whereas for the positively polarized bulk structure, a surface covered with 2/3 ML OH is the most stable, also showing an overpotential of 0.89 V. Notably, when switching the polarization direction during the reaction, the overpotential becomes as small as 0.20 V for the clean surface and 0.23 V for the surface with the OH coverage, which is far below the usual minimum theoretical overpotential for oxides (η = 0.37 V). We show that the reduction in reaction free energy by ferroelectric switching can be performed in a relevant frequency range and outweighs the energetic cost for polarization switching by a factor of 6–12. Our study demonstrates that switching of improper ferroelectricity is a highly promising route to boost the OER activity of oxynitride photocatalytic water splitting electrodes.