<p>The deployment of green hydrogen in the global transition towards sustainable energy systems relies heavily on cost-effective renewables. Regions rich in renewable energy are usually along a seashore, although the direct electrolysis of seawater is still technically challenging. Moreover, the scarcity of freshwater poses significant costs for hydrogen production. Here we describe an electrolysis process with built-in seawater desalination in a porous-solid-electrolyte reactor. A three-chamber cell with bipolar membranes synergizes the acidic hydrogen evolution reaction, the alkaline oxygen evolution reaction and electrodialysis desalination processes. The use of a porous solid electrolyte in the cell facilitates reliable electrochemical equilibrium, resulting in a coupling factor of approximately 100% for electrolysis and desalination, which maintains the high kinetic current of the coupled electrochemical processes for a lower energy penalty. For seawater electrolysis, the optimized catalyst is robust against anodic Cl⁻ corrosion and serves to enhance device reliability. Long-term cell operation with real seawater experienced negligible device degradation over 360 h. The current system can produce approximately 2.1 t of freshwater meeting potable standards for every kilogram of hydrogen generated, thus offering a feasible pathway to harnessing seawater for both clean water and green hydrogen.</p>