Rechargeable sodium–iodine (Na–I₂) batteries are attracting growing attention for grid-scale energy storage due to their abundant resources, low cost, environmental friendliness, high theoretical capacity (211 mAh g⁻¹), and excellent electrochemical reversibility. Nevertheless, the practical application of Na–I₂ batteries is severely hindered by their poor cycle stability owing to the serious dissolution of polyiodide in the electrolyte during charge/discharge processes. Herein, the atomic modulation of metal–bis(dihydroxy) species in a fully conjugated phthalocyanine copper metal–organic framework (MOF) for suppression of polyiodide dissolution toward long-time cycling Na–I₂ batteries is demonstrated. The Fe₂[(2,3,9,10,16,17,23,24-octahydroxy phthalocyaninato)Cu] MOF composited with I₂ (Fe₂–O₈–PcCu/I₂) serves as a cathode for a Na–I₂ battery exhibiting a stable specific capacity of 150 mAh g⁻¹ after 3200 cycles and outperforming the state-of-the-art cathodes for Na–I₂ batteries. Operando spectroelectrochemical and electrochemical kinetics analyses together with density functional theory calculations reveal that the square planar iron–bis(dihydroxy) (Fe–O₄) species in Fe₂–O₈–PcCu are responsible for the binding of polyiodide to restrain its dissolution into electrolyte. Besides the monovalent Na–I₂ batteries in organic electrolytes, the Fe₂–O₈–PcCu/I₂ cathode also operates stably in other metal–I₂ batteries like aqueous multivalent Zn–I₂ batteries. Thus, this work offers a new strategy for designing stable cathode materials toward high-performance metal–iodine batteries.