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Developing monovalent anion-selective membranes (MAPMs) faces challenges, including the trade-off between flux and selectivity, membrane stability, and cost-effective fabrication. Overcoming these requires advanced material design and scalable techniques. Here, we introduce in situ interfacial polymerization (ISIP) to prepare MAPMs. Base membranes are synthesized via superacid polymerization and modified with anion channels and -NH2 groups. During ISIP, trimesoyl chloride reacts with surface -NH2 groups, forming a partially crosslinked structure with -COOH groups to regulate ion transport via electrostatic interactions. This results in low membrane resistance (4.7 Ω cm2) and selective transport of weakly hydrated ions (Cl−, Br−, NO3−), while strongly hydrated ions (SO42−, F−) face higher barriers. MAPMs demonstrate high performance, achieving a limiting current density (>90 mA cm−2), Cl− flux (1.98 mol m−2 h−1 at 5 mA cm−2), and selectivity (244 for Cl−/SO42−), confirming effective hydration dynamics control and balanced performance. Simulations reveal how charge distribution affects ion migration pathways.