This paper reports a versatile seed-mediated growth method for selectively synthesizing single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals. In the seed-mediated growth method, cetylpyridinium chloride (CPC) and CPC-capped single-crystalline gold nanocrystals 41.3 nm in size are used as the surfactant and seeds, respectively. The CPC-capped gold seeds can avoid twinning during the growth process, which enables us to study the correlations between the growth conditions and the shapes of the gold nanocrystals. Surface-energy and kinetic considerations are taken into account to understand the formation mechanisms of the single-crystalline gold nanocrystals with varying shapes. CPC surfactants are found to alter the surface energies of gold facets in the order {100} > {110} > {111} under the growth conditions in this study, whereas the growth kinetics leads to the formation of thermodynamically less favored shapes that are not bounded by the most stable facets. The competition between AuCl4 reduction and the CPC capping process on the {111} and {110} facets of gold nanocrystals plays an important role in the formation of the rhombic dodecahedral {RD} and octahedral gold nanocrystals. Octahedral nanocrystals are formed when the capping of CPC on {111} facets dominates, while RD nanocrystals are formed when the reduction of AuCl4 on {111} facets dominates. Cubic gold nanocrystals are formed by the introduction of bromide ions in the presence of CPC. The cooperative work of cetylpyridinium and bromide ions can stabilize the gold {100} facet under the growth condition in this study, thereby leading to the formation of cubic gold nanocrystals.