A ZnO nanocactus (NC) array is constructed by the chemical bath deposition of hierarchical nanostructures, i.e. shells and spines, on the ZnO nanowire (NW) array at room temperature (RT). By utilizing a supersaturated solution of zinc acetate and NaOH, etch pits are initially formed on the surface of ZnO NWs, where the spines are subsequently developed from. The shell is epitaxially grown on the surface of the primary NW simultaneously. Using the ZnO NC array as the anode of a dye-sensitized solar cell (DSSC), the hierarchical ZnO nanostructures on NWs provide not only larger and more fitting surface for dye adsorption but also faster electron transport pathways compared to the as-prepared ZnO NW anode. Nearly 2-fold enhancement of the efficiency of the ZnO NW DSSC is achieved simply by the formation of the hierarchical nanostructures at RT for 20 min.