This paper reports a facile method to prepare gold nanocubes with an average size length of 13 ± 2 nm, a shape selectivity >90%, and a very good reproducibility. The gold nanocubes were prepared by photocatalytic reduction of Au(III) using a cationic, water soluble porphyrin as the photocatalyst, and cetyltrimethylammonium bromide as the capping agent. The synthesis is performed in water, at pH 7, and room temperature. The nanocubes were characterized by transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and X-ray phototelectron spectroscopy. Shape selectivity for the synthesis of nanocubes is due to a different rate of growth of the (100) and (111) facets of the nanoparticles, and such control in this synthesis is achieved by careful selection of the initial gold precursor for the photocatalytic cycle.