Well-defined and monodisperse dendritic platinum nanoparticles (DPNs) are successfully prepared by a rapid, one-step, and efficient route with high yield in aqueous solution, wherein neither organic solvents nor surfactants are employed, ensuring the as-made DPNs definitely have "clean" surfaces, allowing them to exhibit high activity for both methanol oxidation and p-nitrophenol reduction. This proposed strategy for simple and facile preparation of "clean" metal nanocatalysts paves the way for accurately evaluating and further improving their intrinsic catalytic activity.