Despite the importance of gallium nitride (GaN) nanostructures for photocatalytic activity, relatively little attention has been paid to their geometrical optimization on the basis of wave optics. In this study, we present GaN truncated nanocones to provide a strategy for improving solar water splitting efficiencies, compared to the efficiency provided by the conventional geometries (i.e., flat surface, cylindrical, and cone shapes). Computational results with a finite difference time domain (FDTD) method and a rigorous coupled-wave analysis (RCWA) reveal important aspects of truncated nanocones, which effectively concentrate light in the center of the nanostructures. The introduction of nanostructures is highly recommended to address the strong light reflection of photocatalytic materials and carrier lifetime issues. To fabricate the truncated nanocones at low cost and with large-area, a dry etching method was employed with thermally dewetted metal nanoparticles, which enables controllability of desired features on a wafer scale. Experimental results exhibit that the photocurrent density of truncated nanocones is improved about three times higher compared to that of planar GaN.