Although recent years have witnessed considerable progress in the synthesis of metal clusters, there is still a paucity of reports on photoelectrochemical (PEC) properties of metal cluster/semiconductor systems for solar energy conversion. In this work, highly ordered glutathione (GSH)-protected gold (Au) cluster (Au_x@GSH) enwrapped ZnO nanowire array (NW) heterostructures (Au_x/ZnO NWs) were designed by a facile, green, simple yet efficient in situ etching-induced electrostatic self-assembly strategy by modulating the intrinsic surface charge properties of building blocks, which renders negatively charged Au_x clusters spontaneously and uniformly self-assembles them on positively charged ZnO NWs framework with intimate interfacial integration. It was unraveled that such Au_x/ZnO NWs heterostructures demonstrated significantly enhanced PEC water splitting performance in comparison with single ZnO NWs, Au nanoparticles (Au/ZnO NWs) and GSH-capped Ag_x clusters (Ag_x/ZnO NWs) decorated ZnO NWs counterparts under both simulated solar and visible light irradiation. The vitally important role of Au_x clusters as photosensitizer was unambiguously revealed and the merits of Au_x clusters in boosting charge transfer arising from their unique core–shell architecture were highlighted by systematic comparison under identical conditions, based on which Au_x cluster-mediated PEC water splitting mechanism is delineated. It is anticipated that our work can highlight the possibility of harnessing metal clusters as efficient light-harvest antennas and open new avenues for rational construction of various highly energy efficient metal cluster/semiconductor heterostructures for widespread photocatalytic and PEC applications.