Rechargeable aqueous Zn–I₂ batteries face challenges from zinc anode degradation (dendrites, corrosion) and polyiodide (I₃⁻/I₅⁻) shuttling at the cathode, limiting cycle life. To address these issues simultaneously, a novel ion-selective, solvation-regulating, and flexible sulfonic acid-based water reducer gel electrolyte (polyacrylamide (PAM)-polynaphthalene sulphonate (FDN)-carboxylated chitosan (CCS)/zinc sulfate (ZSO)) is designed in this work. This electrolyte features a three-dimensional (3D) porous structure and abundant polar groups enabling efficient Zn²⁺ transport and solvation structure regulation, promoting uniform zinc deposition and suppressing water-related side reactions (e.g., hydrogen evolution) at the anode. Crucially, the strongly negatively charged sulfonic acid groups impart exceptional ion selectivity: They electrostatically repel polyiodide anion, effectively blocking their shuttle to the anode and minimizing active iodine loss, while permitting unimpeded Zn²⁺ diffusion. Consequently, Zn–I₂ full cells employing this multifunctional gel electrolyte achieve outstanding cycling stability, retaining 118.5 mAh·g⁻¹ after 9000 cycles at 5 A·g⁻¹. This work achieves the synergistic optimization of interface issues in Zn–I₂ batteries by constructing an ion-selective multifunctional gel electrolyte, significantly enhancing their overall electrochemical performance.