<p>Ion exchange membranes play a critical role in environmental sustainability, with applications ranging from separations to energy storage and conversion. However, polymer-based membranes typically suffer from a trade-off between selectivity and conductivity, hindering the development of many membrane-based devices. Here we overcome this trade-off in a proton-exchange membrane (PEM) by enabling hydrogen molecules and electrons to be transported through a gas chamber and a solid metal, respectively, instead of relying on proton conduction. The facile conversion between protons, electrons and hydrogen molecules occurs via hydrogen evolution and oxidation reactions at the triple-phase boundary. This design, which we term hydrogen-molecule-mediated PEM, achieves nearly 100% proton selectivity by eliminating direct ionic pathways between 2 electrolytes. Benefiting from the rapid diffusion of hydrogen gas, the high electrical conductivity of metals, and the fast kinetics of hydrogen evolution and oxidation reactions, hydrogen-molecule-mediated PEM exhibits exceptionally low area-specific resistance of 0.15 Ω cm² at pH 0 and 1.05 Ω cm² at pH 14, comparable to commercial membranes. Finally, we demonstrate its applications in a pH-decoupled Zn-MnO₂ rechargeable battery with stable operation for 200 cycles and an electrochemical stack for efficient acid and base generation from seawater.</p><p></p>