Abstract: Direct seawater electrolysis presents a promising pathway for sustainable “green hydrogen” production. However, the complex composition of seawater, particularly the presence of chloride ions (Cl−), poses significant challenges to the structural stability and electrocatalytic performance of oxygen evolution reaction (OER) catalysts. Although recent studies have demonstrated that anion modification can improve both the stability and activity of catalysts, the extent of these improvements varies considerably across different anions, and the underlying mechanisms remain poorly understood. This review examines the electrochemical behavior of anions related to their physicochemical properties and provides a comprehensive overview of recent advances and remaining challenges in anion‐oriented strategies for seawater electrolysis. First, we propose a novel framework for determining anion properties based on adsorption energy, ionic potential, and acid-base character, which evaluates the physicochemical properties of anions from three dimensions and serves as a guideline for selecting modification materials for catalysts. Second, we critically discuss the underlying mechanisms by which anion modification enhances OER stability and activity in seawater, with a focus on chlorine chemistry and oxygen evolution dynamics. Classical approaches for stability improvement, such as the introduction of external anions and the regulation of Cl− and OH−, are discussed. We also summarize mechanisms for activity enhancement, including electronic structure modulation, active species engineering, and mass transfer optimization. Finally, we outline future research directions for anion modification strategies and highlight persistent challenges.