By effectively bypassing the common problem of the loss of contact of platinum nanoparticles to carbon black support involved in the traditional methodology, supportless Pt, Au@Pt 3D-porous nanostructures (PtNBs and Au@PtNBs), and Aucore-Ptshellnanoparticles (Au50Pt50NPs) were synthesized. The radiolytic synthesis provides a high control on the size, morphology, and composition of the nanoparticles. Nanoballs, 85 ± 5 nm for PtNBs and 75 ± 5 nm for Au@PtNBs, are formed by 3D-interconnected nanowires leading to a giant mesoporous structure. A single Au@PtNB is formed by an ca. 30 nm Au-core, surrounded by a porous Pt-shell of 15 nm thickness made by 3D-connected nanowires of 2 nm diameter. The high catalytic activity of these nanostructures toward organic electrooxidation was highlighted in aqueous media and attributed to their particular core–shell structure. The Au@PtNBs mesoporous materials exhibit improved kinetics toward glucose electrooxidation compared to their counterpart PtNBs and are a promising anode material for direct glucose fuel cells. For formic acid electrooxidation, Au50Pt50NPs (3 nm diameter) exhibit the most specific activity. The radiolysis enables the effective synthesis of various core–shell nanostructures ranging from simple to mesoporous ones. The present work continues the research line where advanced methods are used to prepare highly active nanostructures with improved catalytic kinetics.