Sulfonated multiblock copolymers composed of Polysulfone (PSU) and Polyphenylsulfone (PPSU) poly(ether sulfone) segments (SPSU/SPPSU) are synthesized for the first time by polycondensation in a “one-pot two-step synthesis” of commercial monomers, followed by sulfonation reaction with trimethylsilyl chlorosulfonate (TMSCS). Both segments are responsible for proton conductivity, although the PSU block has greater affinity to be sulfonated. Even though no microphase separation is detected, the resulting ionomers exhibit good mechanical properties due to the non-sulfonated blocks remaining and to the high molecular weights of the ionomers. The chemical structure is confirmed by ¹H-NMR, ¹⁹F-NMR and FTIR analysis. The degree of sulfonation (0.93–1.58) is determined from the IEC values and ¹H-NMR spectra. In situ through-plane proton conductivity measured on the MEAs is 34.1 mS cm⁻¹ at 70 °C. A maximum power density of 400 mW cm⁻², a current density of 1100 mA cm⁻² and outstanding thermo-mechanical stability, these proton-conducting membranes can therefore be implemented in PEMFC.