We report a successful fabrication of hierarchical three-dimensionally branched TiO₂/Si nanowire arrays and their application as the photoelectrochemical electrode for solar water splitting. The branched TiO₂/Si nanowire arrays improved the photoelectrochemical (PEC) performance compared to TiO₂ thin film-coated Si nanowire arrays because of the substantially increased surface area for electrochemical reactions and enhanced charge transfer kinetics. Wavelength-dependent photocurrent response of the branched nanowire array indicates a strong response in the ultraviolet region (<400 nm), while a negligible photocurrent is observed under visible illumination, primarily caused by the high overpotential loss of n-Si photoanode and energy band configuration of the TiO₂/Si heterojunction. The working mechanism based on recombination at this heterogeneous n–n junction is proposed. This study provides insights on the fundamental understanding and potential optimizations of nanoscale hierarchical 3D structured devices for renewable energy applications.