Graphene quantum dot (GQD)-supported platinum (Pt/G) nanoparticles were prepared by a simple hydrothermal procedure at controlled temperatures. Transmission electron microscopic measurements showed that the platinum nanoparticles exhibited a rather consistent core size within the narrow range of 2.5-3.5 nm in diameter (average core diameters slightly lower than 3.0 nm) when the hydrothermal temperatures were varied between 140 and 180 °C, whereas at higher temperatures (200 °C) the nanoparticle core size was markedly larger, as a result of weakened anchoring and passivation of the metal nanoparticles by the diminishment of the GQD structural defects. Spectroscopic measurements based on Fourier-transformed infrared, Raman, and X-ray photoelectron spectroscopies confirmed the formation of various oxygenated structural defects on GQDs and the variation of their concentrations with the hydrothermal conditions. Interestingly, electrochemical studies showed that the electrocatalytic activity of the series exhibited a volcano-shaped variation with the GQD structural defects, with the best identified as the samples prepared at 160 °C for 12 h and at 180 °C for 6 h where the mass activity was found to meet the DOE target for 2017. This remarkable performance was accounted for by the deliberate manipulation of the adsorption of oxygen and reaction intermediates on platinum by the GQD structural defects through partial charge transfer. The strategy presented herein may offer a new paradigm in the design and engineering of nanoparticle catalysts for fuel cell electrochemistry.