A highly sulfonated poly(ether sulfone) with binaphthyl units (BNSH-100) applicable to polymer electrolyte membranes (PEMs) was synthesized by the facile nucleophilic aromatic polymerization of disodium 3,3′-disulfo-4,4′-difluorophenyl sulfone and 1,1′-binaphthyl-4,4′-diol to yield a high-molecular-weight fibrous polymer (η_inh = 1.01 dL g⁻¹) with a high ion exchange capacity (IEC) value (3.01 mequiv g⁻¹). BNSH-100 gave a transparent, flexible, and tough membrane by solution casting. The BNSH-100 membrane demonstrated high mechanical strength in the dry state regardless of its high IEC value. The dimensional changes of the BNSH-100 membrane were acceptable even in the hydrated state (80 °C and 95% RH) considering its high IEC value. The BNSH-100 membrane showed high water uptake (18.7 wt%) at 50% RH and it was still high (11.6 wt%) even at 30% RH. Proton conductivity of the BNSH-100 membrane at 80 °C was higher than that of Nafion 117 in the range of 30–95% RH. Even at 30% RH, the BNSH-100 membrane showed 1.5 times higher proton conductivity (6.3 × 10⁻³ S cm⁻¹) than that of Nafion 117. The high IEC value of BNSH-100 resulted in excellent proton conductivity over a wide range of relative humidities, and the high hydrophobicity of the binaphthyl units contributed to good dimensional stability simultaneously. Finally, the surface morphology of the BNSH-100 membrane was investigated by tapping mode atomic force microscopy (AFM), which showed continuous hydrophilic domains due to its high IEC value. This phase-separated structure derived from its high IEC value contributed to excellent proton conductivity as well.