We report the development of a seed-mediated and iodide-assisted method for the synthesis of monodisperse gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures. Particle growth is complete in 15 min at room temperature, so the process is fast and energy-efficient. By progressively increasing the volume of KI used in a growth solution while keeping the amount of ascorbic acid added constant, nanocrystals with morphologies that vary from rhombic dodecahedral to rhombicuboctahedral, edge- and corner-truncated octahedral, corner-truncated octahedral, and octahedral structures were synthesized. The nanocrystals are monodisperse in size and readily form self-assembled structures on substrates. By simply adjusting the volume of gold seed solution added to a growth solution, particle sizes of the octahedral gold nanocrystals can be tuned with average opposite corner-to-corner distances of 42, 48, 54, 60, 68, 93, 107, and 125 nm. In the presence of HAuCl4, iodide may act as a reducing agent. Variation of its volume in the solution may slightly modulate the reduction rate and affect the final crystal morphology. Intermediate structures collected during crystal growth reveal the presence of many twisted structures that surround a developing nanocrystal core. This nanocrystal growth mechanism and the less important role of surfactant in directing the polyhedral nanocrystal morphology is discussed.