Sulfonated poly(ether sulfone)s (SPESs) are developed which have different sulfonation level as a polymer electrolyte membrane for high‐temperature operation. The sulfonation level of SPESs is calculated by 1H NMR, and the molecular structure and crystalline structure of SPESs are evaluated by Fourier transform infrared and X‐ray diffraction. SPES membranes are thermally stable up to 250 °C. SPES membranes can keep their shapes at 120 °C and 23%RH. Water uptake at 120 °C and 23%RH is 5.7–6.4 wt%, while Nafion 212 shows 2.4 wt%. Proton conductivity measurements of SPESs are carried out from 30 to 120 °C at different relative humidity. With increasing sulfonation level of SPES, proton conductivity increases in all humidity. The proton conductivity obtained from all SPESs is more than 100 mS cm–1 at 120 °C in high humidity (>90%RH), and high‐sulfonation SPES shows higher conductivities than Nafion 212 at 120 °C, 20%RH.