We propose a universal, green, and surfactant-free strategy to synthesize noble metal particles with high monodispersity using gaseous H2 as a reducing agent in a solution at 60 °C. The prepared Pt nanoparticles have a 24 mV more positive half-wave potential than the commercially available Pt/C in the oxygen reduction reaction, while showing high durability with negligible half-wave potential decay after 10 000 cycles of testing.