Titanium oxide is commonly considered as an effective catalyst for photo-electrochemical water splitting due to its low cost, high activity, and perfect stability. However, a wide bandgap of TiO₂ prevents the utilization of visible and near-infrared photons in the photo-electrochemical process. Photosensitization of TiO₂ with plasmon active nanostructures was proposed as a way to solve this problem but the optimal design of coupled TiO₂-plasmonic nanostructures, as well as the exact mechanism of plasmon triggering, are still under debate. In this work, we propose a plasmon-based TiO₂ photosensitization, using the gold grating, covered by homogenous TiO₂ layer. The gold grating supports the excitation and propagation of surface plasmon polariton wave (SPP), which is responsible for TiO₂ triggering (unlike common localized plasmon resonance commonly used for TiO₂ activation). For the optimization of the present structure design, we studied the impact of TiO₂ layer thickness, illumination regime, and spatial distribution of plasmonic evanescent wave energy with regards to the mechanism of TiO₂ activation. Obtained results proved that the proposed structure can be used for efficient photo-electrochemical water splitting and hydrogen production under irradiation with NIR photons of 700–1000 nm wavelengths. It was also verified that for a certain TiO₂ layer thickness the activation of catalytic activity is mostly due to the plasmon electric field, which is concentrated at the electrolyte/catalyst surface.