A facile synthetic strategy was developed for a hollow Ru octahedral nanocage via an in situ generated metastable facet-controlled Cu nanooctahedron. Co-decomposition of Cu and Ru precursors forms a metastable core-shell Cu@Ru nanoparticle, and a subsequent in situ dissolution of the core Cu phase results in a hollow octahedral Cu-doped Ru nanocage. CTAB (cetyltrimethylammonium bromide) was found to effect both the formation and destabilization of the Cu template under the employed reaction conditions, which is the key requirement for one-step synthesis of hollow Cu-doped Ru nanocages. The regioselective, preferential deposition of Ru atoms on the edge and corner of the core template nanoparticle led to a structurally well-defined Cu doped-Ru octahedral nanocage.