The influence of nanostructuring on the photoelectrochemical (PEC) properties of GaN is investigated. GaN nanopillar arrays are fabricated by inductively-coupled-plasma dry etching of a GaN epitaxial layer, using a self-assembled Ni cluster mask. Pillars of 0.4–1.6 μm in height were prepared and were investigated photoelectrochemically. After nanoroughening, the surface area increases up to 6 times and the plateau photocurrent increases by 84% with respect to planar GaN. The pillar structure provides abundant depletion area and therefore enhances the photocarrier separation. Surface recombination becomes more important after the dry etching process, as confirmed by the PEC and photoluminescence measurements.