The electrical performance of a proton exchange membrane fuel cell (PEMFC) is limited by the slow oxygen reduction reaction (ORR) kinetics. Catalytic improvements for the ORR have been obtained on alloyed PtM/C or M-rich core@Pt-rich shell catalysts (where M is an early or late transition metal) compared to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure on the ORR kinetics remains under investigated. In this study, nanometre-sized PtNi/C electrocatalysts with low Ni content (~15 at. %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow or “sea sponge” PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning-transmission electron microscopy (TEM and STEM, respectively), X-ray energy dispersive spectroscopy (X-EDS) Synchrotron wide-angle X-ray scattering (WAXS), atomic absorption spectroscopy (AAS) and electrochemical techniques. Their electrocatalytic activities for the ORR were determined, and structure-activity relationships established. The results showed that (i) the compression of the Pt lattice by ca. 15 at. % Ni provides mild ORR activity enhancement compared to pure Pt/C, (ii) highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a commercial Pt/C material with similar crystallite size, (iii) the enhancement in ORR catalytic activity can be ascribed to the presence of structural defects as shown by two independent parameters: the microstrain determined from WAXS, and the average COads electrooxidation potential (µ_1^CO) determined from COads stripping measurements. This work indicates that, at fixed Ni at. %, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono or bimetallic nanocatalysts.