We here report a new sulfonated poly(arylene ether sulfone) modified with hydroxyl side groups for a proton exchange membrane fuel cell (PEMFC). The polymer was synthesized via 4-step reaction including direct copolymerization, sulfochlorination, demethylation, and hydrolysis. By adding the hydroxyl group into the polymer backbone, strong intermolecular hydrogen bonds could be generated between polymer chains. The hydroxyl sulfonated polymer demonstrated high proton conductivities at a variety of temperatures and relative humidities due to its enhanced phase-separated morphology. Moreover, it showed an improved fuel cell performance compared to the non-hydroxylated analog with the same sulfonation degree. The introduction of hydroxyl groups to the sulfonated poly(arylene ether sulfone) provides a potential candidate for an improved PEMFC membrane.