Ferroelectric materials, such as BaTiO₃ or BiFeO₃, retain an electric polarization after exposure to an external electric field. In semiconductor photoelectrodes, such a ferroelectric effect (FE) could be potentially used to enhance the carrier separation and photoelectrochemical activity under photoexcitation. However, to date, examples of FE-enhanced photoelectrochemistry are rare. Here, we report the discovery of a FE effect in oxygen vacancy rich SrTiO₃ nanoparticles and single crystals, and their application for improved photoelectrochemical water oxidation. Single crystals terminated by [111] facets were subjected to hydrogen annealing at 1000 C to induce oxygen vacancies, as confirmed with optical absorption and X-ray photoelectron spectroscopy. In 0.5 M aqueous Na₂SO₄ solution 1.0 cm² large crystals produce a 2.1 mA photocurrent at 0.6 V RHE and under 55 mW cm^-2 UV illumination. Prior exposure to 10 kV cm-1 electric field increases the photocurrent to 4.5 mA or decreases it to 0.05 mA, depending on the orientation of the polarization. The effect of the electric polarization on the carrier separation direction is confirmed with surface photovoltage spectra. At room temperature, the FE in SrTiO₃ is relatively short lived; samples stored in argon retain 85% of the FE effect after 24h, but samples stored in air loose 100% of the FE, likely due to oxygen uptake and disappearance of the vacancies. To obtain a microscopic interpretation of the observed FE effect we investigated ferroelectricity in oxygen-deficient SrTiO₃ by density functional theory with effective many-body corrections. These calculations applied to SrTiO₃ bulk and surfaces provide a rationale to interpret the experimental results. The finding of an FE in oxygen vacancy rich SrTiO₃ is significant because it may be applicable to other metal oxides in the cubic perovskite structure type.