A facile one step route has been developed for the synthesis of trimetallic Cu@RhRu core-shell concave nanooctahedra by co-decomposition of Ru, Rh and Cu precursors. A mechanistic study reveals that nanoparticles with a CuRh alloy core and a Ru shell are initially formed and a subsequent migration of Rh to the shell results in the Cu@RhRu core-shell concave nanooctahedron. The shell exhibits atomically mixed Ru and Rh phases with an fcc atomic structure, although the hcp atomic structure is commonly found for the bulk Ru. We also report an unusually high catalytic activity of the Cu@RhRu octahedral nanocrystals toward the oxygen evolution reaction in alkaline solution.