C-doped ZnO hierarchically porous nanoarchitectures were synthesized in situ on indium tin oxide (ITO) through a counter strategy. The PEC performance of the C-doped ZnO nanoarchitectures in the splitting of water without sacrificial reagents was systematically evaluated for the first time. In comparison to other ZnO-based photoanodes in the literature, C-doped ZnO nanoarchitectures exhibit a striking photoresponse. Not only do they have a maximum IPCE value of 95%, but they also have an IPCE at the monochromatic wavelength of 400 nm as high as 26.6%, implying that modification by doping with carbon substantially improves the light utilization and conversion efficiency in the visible region of interest over those obtained using a conventional ZnO structure. This model hybrid photoanode will enable us to design high-activity, high-stability visible-light-driven photoelectrodes in the future.