A nanocomposite material consisting of platinum nanoparticles surrounded by an ionic conducting polymer dispersed on carbon Vulcan XC72 was synthesized. The aim of this nanocomposite material is to translate the triple-phase boundary to a molecular level in electrochemical systems involving a polymer electrolyte. The ionic conducting polymer is a poly(styrenesulfonic acid) (PSSA, or PSSNa in its sodium form) synthesized by atom-transfer radical polymerization. The polymer has a terminal thiol group to ensure bonding with platinum nanoparticles. The nanocomposite material (Pt-PSSA/C) exhibited thermal stability up to 160 °C and electrochemical stability up to 1 V versus RHE. Compared to a Pt/C catalyst, the nanocomposite catalyst has a lower active surface area but comparable catalytic activity for the oxygen reduction reaction. Furthermore, this nanocomposite material exhibits similar behavior in a fuel cell active layer without Nafion as a classical Pt/C catalyst with Nafion included in the active layer.