This work presents simple post-treatment methods to selectively and partially remove the Pd core of Pd–Pt core–shell (Pt@Pd/C) catalysts. The proton exchange membrane fuel cell with the post-treated Pt@Pd/C cathode (Pt loading: 0.10 mg∙cm⁻²) delivers an impressive peak power density of 1.2 W∙cm⁻². The partial removal of Pd core endows an ultrahigh oxygen reduction reaction (ORR) mass activity of 0.32 A∙mg_PGM⁻¹ when normalized to the platinum group metal (PGM) mass, or equivalently 0.55 A∙mg_Pt⁻¹ at 0.9 V measured in a fuel cell. The post-treatment thickens the Pt shells and mitigates the Pd dissolution during potential cycling. As a result, the post-treated core–shell catalyst demonstrates superior durability in ORR mass activity and polarization power density retention than untreated core–shell catalyst and benchmark Pt/C. In-situ inductively coupled plasma-mass spectrometry (ICP-MS) results highlight that the amount of dissolved Pd in post-treated core–shell catalyst is 17-times lower than that of the untreated one. Our findings highlight the importance of structural tuning of catalysts in enhancing their mass activity and durability.