Photocatalysis, which utilizes solar energy to electrochemically decompose water pollutants into harmless products, has attracted considerable attention to address serious environmental issues. The photocatalytic effect can be enhanced using an external electric field owing to the inhibition of the recombination of photoexcited electrons and holes. However, the typical linear potential bias that induces a small potential drop across a thin photocatalyst film exhibits a limited photocatalytic reaction. Herein, we propose an ion-concentration-polarization-assisted photocatalytic reactor that generates a nonlinear electric field across the microchannel of this system, which enables an 85.5% increase in the reaction rate compared to that achieved using a linear potential, and a high reaction rate constant up to 12.7 min⁻¹ is achieved. The nonlinear electric field induced by concentration polarization, the nanofluidic electrokinetic phenomenon, results in a considerably increased potential drop across the photocatalyst layer such that the recombination of photoexcited electrons and holes may be efficiently prevented. The facilitated photocatalytic reaction is verified with the plastic film degradation. This proposed enhancing mechanism shows a novel application of nanofluidics for improving the photocatalytic effect, and the potential to be a new class of platform for a photocatalytic reactor owing to its simple configuration and fabrication procedures.