<p>Aqueous Zn-I flow batteries are attractive for grid storage owing to their inherent safety, high energy density, and cost-effectiveness. However, Zn anode deposition/dissolution reactions cause severe water migration owing to ionic imbalance, especially under harsh conditions with high state-of-charge and high areal/volumetric capacities, further exacerbating intrinsic challenges for practical Zn-I systems. Herein, we develop a tailored ionic-molecular sieve membrane to regulate the transport behaviors of water/hydrated ion clusters, enabling the electrolyte balance by precise size sieving effects. Systematic investigations of different subnanometer pore sizes reveal that the optimal range (0.55–0.65 nm) can selectively intercept large hydrated ion clusters and reduce polyiodide shuttling. In this way, Zn-I flow batteries with this membrane exhibit a stable cycling over 2000 h (500 cycles) under harsh conditions (50% state-of-charge), achieving 66.4 mAh cm⁻²/53.2 Ah L⁻¹_posolyte/27.66 Wh L⁻¹_system. This systems also deliver a low self-discharging rate, retaining a high Coulombic efficiency of 98.5% after 3 days static flow. Furthermore, techno-economic cost analysis reveals a competitive levelized cost of long-term energy storage for systems incorporating this membrane (551.98 USD MWh⁻¹ at an energy-to-power ratio of 18 h). This work offers insights into controlling water transport behaviors for realizing long-life flow batteries.</p>