Carbon-supported Pt@Cu “core−shell” nanoparticles with Pt−Cu alloy core and Pt shell have been synthesized by a galvanic displacement of Cu by Pt4+ at ambient conditions, followed by a leaching out of unreacted Cu on the surface by treating with 9 M H2SO4. X-ray diffraction (XRD) data indicate the formation of a Pt−Cu alloy below the Pt shell. Energy dispersive spectroscopic (EDS) analysis in a scanning electron microscope (SEM) reveals that the experimental Cu content is much lower than the initial nominal Cu content, confirming the displacement of a significant amount of Cu by Pt. X-ray photoelectron spectroscopic (XPS) studies indicate surface enrichment by Pt. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements demonstrate an enhanced catalytic activity for the oxygen reduction reaction (ORR) for optimum Pt@Cu compositions compared to that found with commercial Pt catalyst, both per unit mass of Pt and per unit active surface area basis. Moreover, the surface area specific activities of the Pt@Cu samples increase linearly with increasing initial nominal Cu content. The increase in activity for ORR is ascribed to an electronic modification of the outer Pt shell by the Pt−Cu core.