Seawater electrolysis offers a promising route to low-cost, large-scale green hydrogen production without competing for precious freshwater resources. However, it faces two major challenges: the chlorine evolution reaction (CER) at the anode and Mg²⁺/Ca²⁺ precipitation at the cathode. To address these challenges, this work constructs a pH-asymmetric seawater electrolyzer based on a bipolar membrane (BPM), feeding acidified seawater to the cathode and alkaline freshwater to the anode: acidification prevents Mg²⁺/Ca²⁺ precipitation at the cathode, while the BPM blocks chloride ions and thereby prevents CER at the anode. Isotope labeling experiments confirm that the water molecules dissociated within the BPM originate mainly from the cathodic seawater; moreover, the electrolyzer remains stable even when the cathode is fed with saturated seawater. By optimizing the electrolyte concentration and electrode types, this seawater electrolyzer achieves continuous, stable operation for 500 h at a current density of 100 mA∙cm⁻², with a voltage rise rate of only 1.76 mV∙h⁻¹, no cathodic scaling, and no anodic chlorine evolution. This work offers valuable insight into advancing seawater electrolysis for hydrogen production.