The use of vast amounts of high-purity water for hydrogen production may aggravate the shortage of freshwater resources. Seawater is abundant but must be desalinated before use in typical proton exchange membrane (PEM) electrolysers. Here we report direct electrolysis of real seawater that has not been alkalised nor acidified, achieving long-term stability exceeding 100 h at 500 mA cm‾² and similar performance to a typical PEM electrolyser operating in high-purity water. This is achieved by introducing a Lewis acid layer (for example, Cr₂O₃) on transition metal oxide catalysts to dynamically split water molecules and capture hydroxyl anions. Such in situ generated local alkalinity facilitates the kinetics of both electrode reactions and avoids chloride attack and precipitate formation on the electrodes. A flow-type natural seawater electrolyser with Lewis acid-modified electrodes (Cr₂O₃–CoO₊) exhibits the industrially required current density of 1.0 A cm‾² at 1.87 V and 60 °C. ; Jiaxin Guo, Yao Zheng, Zhenpeng Hu, Caiyan Zheng, Jing Mao, Kun Du, Mietek Jaroniec, Shi-Zhang Qiao, Tao Ling