Hydrogen, a zero-emissions fuel, is increasingly being targeted as a source of renewable energy. However, hydrogen production by electrolysis is limited by the characteristics of the hydrogen-separating membrane. Here, a series of multiblock copolymers composed of a hydrophilic block based on disulfonated bis(phenyulfonyl-phenyl) sulfone groups, and a hydrophobic block based on bis(p-phenoxyphenyl) ether groups is developed for application as a proton exchange membrane for water electrolysis. The multiblock copolymers combining stiff hydrophilic block and flexible hydrophobic block are expected to have reduced water swelling and low hydrogen permeability even under full-wet and high-temperature conditions in water electrolysis. Microscopy and X-ray scattering analyses of the multiblock copolymer-based membranes reveal strong nanophase separation and well -developed ion transport channels. The as-synthesized membranes exhibit advantages including low swelling, high proton conductivity, good mechanical and thermal properties, high oxidative stability, and reduced hydrogen permeability. A membrane electrode assembly employing the multiblock copolymer membrane ach-ieved a current density above 3 A cm-2 at 1.8 V, far surpassing that of Nafion 212, and a long-term operating stability exceeding 500 h, demonstrating the suitability of the membrane for practical water electrolysis.