<p>Artificial ion channels have attracted considerable attention in the fields of ion separation, energy conversion, and biosensing. Coupling ion channel-derived functions with spin arrangement in crystals can enable control of magnetic exchange interactions. This study focuses on Na(dibenzo[18]crown-6)[Ni(dmit)₂](CH₃CN)₂ (<b>1</b>), which comprises [Ni(dmit)₂] units with <i>S</i> = 1/2 and a supramolecular cation channel formed by the one-dimensional arrangement of dibenzo[18]crown-6 including Na⁺. When <b>1</b> was immersed in aqueous solutions containing alkali metal cations, the Na⁺ ions in the crystal were replaced by K⁺, Rb⁺, or Cs⁺ ions from the solution while maintaining crystallinity. This ion-exchange process was accompanied by the release of CH₃CN molecules from the crystal and the uptake of water molecules from solution. Furthermore, although the temperature-dependent magnetic susceptibility of <b>1</b> shows ferromagnetic behaviour (Curie–Weiss model: <i>θ</i> = +0.05 K), those of the ion-exchanged crystals were reproduced by the dimer model (K⁺, Rb⁺: dimer model, <i>J</i>/<i>k</i>_B = −797 and −862 K, respectively) and the Curie–Weiss model (Cs⁺:<i>θ</i> = −31.2 K). These results provide a potential approach for controlling magnetic properties through solid-state ion exchange and provide important insights for the development of ion-responsive magnetic sensors and low-dimensional magnetic materials.</p> Graphical abstract <p>Ion exchange through the dibenzo[18]crown-6 channels in Na(dibenzo[18]crown-6)[Ni(dmit)₂](CH₃CN)₂ replaces Na⁺ with K⁺, Rb⁺, or Cs⁺ ions while preserving crystallinity. This process involves acetonitrile release and water uptake, converting ferromagnetic interactions into antiferromagnetic interactions between [Ni(dmit)₂]⁻ anions.</p>