Composite membranes based on different wt percentages of meso-tetrakis-(4-sulfonatophenyl)porphyrin (TPPS) embedded in a medium sulfonation degree (50%) sulfonated poly(etheretherketone) (s-PEEK) were investigated. The successful introduction of porphyrin into the membranes and the characterization of its different species into the membrane ionic domains were carried out by spectroscopic techniques. Moreover, the effect of TPPS arrangement was investigated in terms of water retention, proton conductivity and fuel cell performance at low relative humidity (RH). It was found that the introduction of this porphyrin induces a variation of the chemical-physical parameters, such as ion exchange capacity (IEC), water up-take (Wup %) λ and proton concentration ([H+]), attributable to the interactions that occur between the sulfonic groups of the polymer and the nitrogen sites of TPPS. The TPPS, in its J-aggregated form, actively participates in the proton conduction mechanism, also maintaining the adequate water content in more drastic conditions (80 °C and 50% RH). A maximum power density value of 462 mW cm−2 was obtained for the s-PEEK membrane, with a 0.77 wt % content of TPPS. This evidence suggests that the presence of J-aggregates in the proton conduction channels maintains a good hydration, even if a drastic reduction of the RH of the reactant gases occurs, preventing the membrane from a dry-out effect.