Platonic noble metal nanocrystals (NCs) have attracted considerate attention due to their symmetry, aesthetic beauty, and potential applications in catalysis, plasmonics, sensing, and spectroscopy. Although Platonic noble metal NCs with tetrahedral, cubic, octahedral, and icosahedral geometries have been chemically synthesized, the growth of Platonic dodecahedral noble metal NCs remains elusive. Here we propose a crystal structure of Platonic dodecahedral noble metal NCs and show that via a tailored seed-mediated synthetic approach, Platonic dodecahedral Au NCs can be grown from icosahedral multiply twinned Au seeds. By systematically tuning the ratio between {111} and {110} facets grown on the icosahedral Au seeds, NCs with icosahedral, icosidodecahedral, and dodecahedral shapes can be obtained. These shapes represent a family of Au NCs with icosahedral (I-h) symmetry.