Control over composition and structure on the nanoscale level is critical for designing highly active and durable catalyst to implement in electrochemical energy conversion. Herein, we report a facile strategy for an efficient synthesis of trimetallic PtPdRu dendritic nanocages with hollow cavity and porous dendritic shell by eroding the interior of the starting PtPdRu nanodendrites in acidic solution. The newly discovered trimetallic dendritic nanocages with an open-framework surface afford 3D molecular accessibility and can be used as highly active and durable catalysts for oxygen reduction reaction due to the synergetic effect derived from their unique porous structure and multimetallic composition.