Core-shell nanowire (NW) arrays, which feature a vertically aligned n-type Si NW core and a p-type α-Bi₂O₃ shell, are developed as a highly efficient photoanode that is suitable for water splitting. The morphology and structure of the heterostructure were characterized by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD). The deposition of Bi₂O₃ nanolayers on the surface of the smooth Si NWs causes the surface of the NWs to become rough. The as-prepared core-shell NW photoelectrode has a relatively low reflectance in the visible light region, suggesting good light absorption. The core-shell NW arrays show greatly improved photoelectrochemical water-splitting performance. Photoelectrochemical stability for over 16 h under constant light illumination and fixed bias potential was achieved, illustrating the good stability of this core-shell NW photoanode. These Si/Bi₂O₃ core-shell NW arrays effectively combine the light absorption ability of the Si NWs and the wide energy gap and chemical stability of Bi₂O₃ for water splitting. This study furthers the attempts to design photoanodes from low-cost, abundant materials for applications in water splitting and photovoltaics.