Photocatalytic overall seawater splitting represents a sustainable route to hydrogen production but is fundamentally limited by chloride-induced catalyst corrosion and competing chlorine evolution reactions. Polymeric photocatalysts are attractive for this process; however, intrinsic chlorine tolerance remains elusive. Here we report that poly (triazine imide) single crystals with exposed {1010} facets enable efficient and stable photocatalytic overall seawater splitting. A surface-bound Cl^- layer forms an electrostatic shield that suppresses chloride corrosion, while preferential Na⁺ adsorption generates a strong interfacial electric field that enhances charge separation and extends carrier lifetimes. Consequently, continuous H₂ and O₂ evolution with a stoichiometric 2:1 ratio and without detectable chlorine by-products is achieved under irradiation in natural seawater. These results reveal an ion–framework cooperative mechanism and establish a feasible design principle for chlorine-tolerant polymeric photocatalysts.