While recent advancements in electrocatalysts have led to significant progress toward the commercialization of electrochemical energy conversion devices, performance degradation derived by airborne impurity remains a critical challenge to address. In particular, chloride (Cl^-) poisoning of platinum (Pt) catalysts remains a critical challenge for performance. Herein, an effective strategy for suppressing Cl^- poisoning from the perspective of ionomer layer engineering is demonstrated. From the hybrid interface of cation and anion exchange ionomers, the local microenvironment at the catalyst surface is modified, resulting in significant suppression of Cl^- poisoning. In situ inductively coupled plasma-mass spectrometry analysis revealed that the local Cl^- concentration at the Pt surface decreased by 40% compared to the bulk concentration. These findings highlight the synergistic role of the hybrid ionomer interface in suppressing Cl^- poisoning, validating its effectiveness in maintaining activity and mitigating Pt dissolution. This ionomer engineering approach provides a promising pathway for improving the reliability of electrocatalytic systems under challenging operational conditions.