Highly branched ultrathin Pt-Ru nanodendrites (NDs) with an average thickness of ∼1.8 nm and controlled elemental ratios were prepared through nanoconfined and epitaxial growth inside lamellar micelles of designed surfactants. The elemental compositions could be easily extended to other metals (Pt-M, M = Rh, Co, Fe, Ag, etc.). Owing to their synergistically structural and compositional advantages, Pt-Ru NDs with a specific atomic ratio (specifically Pt90Ru10) showed enhanced electrocatalytic activity and stability toward the methanol oxidation reaction (MOR) under alkaline conditions.