Photoanodes based on In2S3/ZnO heterojunction nanosheet arrays (NSAs) have been fabricated by atomic layer deposition of ZnO over In2S3 NSAs, which were in situ grown on fluorine-doped tin oxide glasses via a facile solvothermal process. The as-prepared photoan-odes show dramatically enhanced performance for photo-electrochemical (PEC) water splitting, compared to single semiconductor counterparts. The optical and PEC proper-ties of In2S3/ZnO NSAs have been optimized by modu-lating the thickness of the ZnO overlayer. After pairing with ZnO, the NSAs exhibit a broadened absorption range and an increased light absorptance over a wide wavelength region of 250-850 nm. The optimized sample of In2S3/ZnO-50 NSAs shows a photocurrent density of 1.642 mA cm-2 (1.5 V vs. RHE) and an incident photon-to-current efficiency of 27.64％ at 380 nm (1.23 V vs. RHE), which are 70 and 116 times higher than those of the pristine In2S3 NSAs, respectively. A detailed energy band edge analysis reveals the type-II band alignment of the In2S3/ZnO heterojunction, which enables efficient separation and collection of photogenerated carriers, especially with the assistance of positive bias potential, and then results in the significantly increased PEC activity.