As a well-known semiconductor that can catalyse the oxygen evolution reaction, TiO₂ has been extensively investigated for its solar photoelectrochemical water properties. Unmodified TiO₂ shows some issues, particularly with respect to its photoelectrochemical performance. In this paper, we present a strategy for the controlled deposition of controlled amounts of GaO_xN_y cocatalysts on TiO₂ 1D nanowires (TiO₂@GaO_xN_y core–shell) using atomic layer deposition. We show that this modification significantly enhances the photoelectrochemical performance compared to pure TiO₂ NW photoanodes. For our most active TiO₂@GaO_xN_y core–shell nanowires with a GaO_xN_y thickness of 20 nm, a photocurrent density up to 1.10 mA cm⁻² (at 1.23 V vs. RHE) under AM 1.5 G irradiation (100 mW cm⁻²) has been achieved, which is 14 times higher than that of unmodified TiO₂ NWs. Furthermore, the band gap matching with TiO₂ enhances the absorption of visible light over unmodified TiO₂ and the facile oxygen vacancy formation after the deposition of GaO_xN_y also provides active sites for water activation. Density functional theory studies of model systems of GaO_xN_y–modified TiO₂ confirm the band gap reduction, high reducibility and ability to activate water. The highly efficient and stable systems of TiO₂@GaO_xN_y core–shell nanowires with ALD deposited GaO_xN_y demonstrate a good strategy for the fabrication of core–shell structures that enhance the photoelectrochemical performance of readily available photoanodes.