Small-sized ordered Pt-based alloys serve as the key catalysts for breaking the activity-stability trade-off in cathodic oxygen reduction reaction (ORR), yet are hampered by facile demetallation of 3d transition metals in harsh environments, as well as site-blocking effects and side reactions from unfavorable reaction pathways. Here we design an ultra-small ordered Pt3In alloy (2.7 nm) confined on defect-rich carbon (Pt3In/DC). This design exploits the low melting point of post-transition metal In and strong PtIn electronic coupling, which grants the catalyst its high structural and chemical durability without sacrificing high activity. The Pt3In/DC catalyst delivers a high mass activity of 1.36 A mgPt−1 at 0.9 V in acidic electrolyte (7.1-fold that of commercial Pt/C), with only a 4 mV half-wave potential (E1/2) decay and 96.2 % mass activity retention after accelerated stability testing. In-situ spectroscopy and density functional theory (DFT) calculations reveal that PtIn dual-site O2 adsorption shifts the ORR from an associative to a direct dissociative pathway, suppressing H2O2 formation and accelerating OH⁎ protonation to mitigate site-blocking effects. This work establishes a scalable route to high-performance ordered Pt-based alloys, advancing proton exchange membrane fuel cell commercialization.