Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA+, the surface self-diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.