Layered oxyhalides with a Sillén(−Aurivillius) structure have attracted considerable attention as promising photocatalysts for efficient visible-light-induced water splitting due to the elevation of the O-2p valence band maximum (VBM) derived from the strong s–p interaction between Pb/Bi-6s and O-2p electrons and the electrostatic destabilization of oxygen anions in the fluorite layers. However, the role of the halogen layer, consisting of single or double layers, was not known. In this study, we examined a Sillén-type layered oxyhalide PbBi3O4X3 (X = Cl, Br) with alternating stacks of single/double halogen layers and skeletal Bi-based oxide layers. Density functional theory (DFT) calculations of PbBi3O4X3 revealed that the two inequivalent halogen layers contribute differently to the valence band density of states; the halogen p orbitals in double layers are higher in energy and responsible for the VBM, which is understood in terms of Madelung site potentials, where the electrostatic repulsion between halide ions is at play. Another finding from the DFT calculation is the spatial separation of the conduction band minimum and the VBM based on the asymmetry of halogen layers, which may promote the charge separation. While both PbBi3O4Cl3 and PbBi3O4Br3 possess band levels appropriate for visible-light-induced water splitting, only the former showed photocatalytic O2 evolution activity. The stability of PbBi3O4Cl3 is due to the significant contribution of the O-2p orbitals to its VBM, in stark contrast to PbBi3O4Br3, where Br-4p predominately contributes to the VBM. PbBi3O4Cl3 functions as a stable O2-evolving photocatalyst in Z-scheme water splitting under visible light irradiation. These results show that the electrostatic repulsion of halogen layers is a key factor for tuning the valence band structure of oxyhalides while maintaining sufficient stability for water-splitting photocatalysis.