Metal nanocrystals (NCs) comprising rhodium are heterogeneous catalysts for CO oxidation, NO reduction, hydrogenations, electro-oxidations, and hydroformylation reactions. It has been demonstrated that control of structure at the nanoscale can enhance the performance of a heterogeneous metal catalyst, such as Rh, but molecular-level control of NCs comprising this metal is less studied compared to gold, silver, platinum, and palladium. We report an iodide-mediated epitaxial overgrowth of Rh by using the surfaces of well-defined foreign metal crystals as substrates to direct the Rh surface structures. The epigrowth can be accomplished on different sizes, morphologies, and identities of metal substrates. The surface structures of the resulting bimetallic NCs were studied using electron microscopy, and their distinct catalytic behaviors were examined in CO stripping and the electro-oxidation of formic acid. Iodide was found to play a crucial role in the overgrowth mechanism. With the addition of iodide, the Rh epigrowth can even be achieved on gold substrates despite the rather large lattice mismatch of ~7%. Hollow Rh nanostructures have also been generated by selective etching of the core substrates. The new role of iodide in the overgrowth and the high level of control for Rh could hold the key to future nanoscale control of this important metal's architecture for use in heterogeneous catalysis.