We present a method for the sequential fabrication of a three-dimensional hierarchical structure, comprising ZnO nanowires (NWs) grown atop Vapor-Liquid-Solid (VLS) GaN NWs using Metal Organic Chemical Vapor Deposition (MOCVD) for applications in photoelectrochemical water splitting (PEC-WS). To enhance the PEC performance and photostability of the system, we performed NiS anchoring. The optimized 60 NiS/ZnO-GaN hierarchical nanowires (HNWs) exhibited a remarkable 23-fold enhancement (1.62 %) in photoconversion efficiency at 0.75 V vs reversible hydrogen electrode (RHE) compared to GaN NWs passivated with ZnO deposited by atomic layer deposition (ALD) (0.07 %), alongside an 18.9-fold increase in photocurrent density (7.6 mAcm−2 @ 1.23 V vs RHE). Additionally, these hierarchical structures demonstrated exceptional photostability over a period of 10 hours (36000 s) at 0.8 V vs RHE. The hierarchical assembly is crucial in achieving high photoconversion efficiency and photostability. The rational design of hierarchical NW photoanodes synergistically enhances both photocurrent and photostability, paving the way for facile integration of II-VI with III-V materials and potentially unlocking advancements in next-generation energy harvesting and conversion devices.