A platinum-catalyzed polyelectrolyte porous membrane was prepared by solid-state compression of electrospun polystyrene (PS) fibers and in-situ metallization of counter-balanced ionic metal sources on the polymer surface. Using this ion-exchange metal-polymer composite system, fiber entangled pores were formed in the interstitial space of the fibers, which were surrounded by sulfonic acid sites (SO3-) to give a cation-selective polyelectrolyte porous bed with an ion exchange capacity (IEC) of 3.0 meq/g and an ionic conductivity of 0.09 S/cm. The Pt loading was estimated to be 16.32 wt% from the SO3- ions on the surface of the sulfonated PS fibers, which interact with the cationic platinum complex, Pt(NH3)42+, at a ratio of 3:1 based on steric hindrance and the arrangement of interacting ions. This is in good agreement with the Pt loading of 15.82 wt% measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The Pt-loaded sulfonated PS media showed an ionic conductivity of 0.32 S/cm. The in-situ metallized platinum provided a nano-sized and strongly-bound catalyst in robust porous media, which highlights its potential use in various electrochemical and catalytic systems.