In this report, we examine the composition- and size-dependent performance in hierarchical Pd1–x Au x nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd1–x Au x ). The ultrathin Pd1–x Au x NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd9Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm2), as compared with elemental Pd NW/C (0.12 mA/cm2) and Pt nanoparticles (NP)/C (0.20 mA/cm2), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd1–x Au x NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd9Au NWs (0.98 mA/cm2, 2.54 A/mgPt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance.