A facile in situ assembly strategy was developed for the fabrication of Pt–Au alloy nanoparticles (NPs) on nitrogen-doped graphene (N–G) sheets, and the as-fabricated Pt–Au/N–G nanocomposites were suitable for electrochemical applications. As characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis and inductively coupled plasma-atomic emission spectroscopy techniques, Pt–Au alloy NPs with an average size of 4–5 nm were uniformly distributed on the N–G surface through intrinsic covalent bonds. The Pt–Au/N–G nanocomposites exhibited excellent electrocatalytic activity and stability towards the methanol oxidation reaction with the highest capability observed for a Pt/Au atomic ratio of 3/1. The unique electrochemical features are distinctive from those of N-free nanocomposites and commercially available Pt/C catalysts, indicative of the alloying effect of Pt–Au and their synergistic interaction with the N–G sheet, which may open up new possibilities for the preparation of N–G-based nanocomposites for other intensive applications as well.