Synthesis of uniform polyhedral gold nanoparticles (Au NPs) by the seed-mediated method is often limited by the difficulties in preparing uniform seeds. Here, we report a facile one-pot synthesis of highly monodisperse single-crystalline spherical Au NPs, which can be used as universal seeds. This method only involves simple mixing of cetyltrimethyl ammonium 4-vinylbenzoate, HAuCl4, AgNO3, and HCl in water at a fixed temperature. The single crystallinity of particles is achieved by the interplay between oxidative etching and controlled surface capping by silver atoms. As-synthesized Au NPs show uniform single crystallinity, shape yield of nearly 100%, and size polydispersity less than 5%. The as-synthesized single-crystalline Au NPs are used as universal seeds to grow monodisperse polyhedral Au NPs of different shapes including octahedra, cubes, rhombic dodecahedra, concave cubes, and concave rhombic dodecahedra with size polydispersity as small as 1.5-4.2% depending on the particle shape, the smallest values for any shape reported so far. This clearly shows the importance of seed uniformity achieved by the present method.