Cation-uptake has been long researched as an important topic in materials science. Herein we focus on a molecular crystal composed of a charge-neutral polyoxometalate (POM) capsule [Mo^VI₇₂Fe^III₃₀O₂₅₂(H₂O)₁₀₂(CH₃CO₂)₁₅]³⁺ encapsulating a Keggin-type phosphododecamolybdate anion [α-PMo^VI₁₂O₄₀]³⁻. Cation-coupled electron-transfer reaction occurs by treating the molecular crystal in an aqueous solution containing CsCl and ascorbic acid as a reducing reagent. Specifically, multiple Cs⁺ ions and electrons are captured in crown-ether-like pores {Mo^VI₃Fe^III₃O₆}, which exist on the surface of the POM capsule, and Mo atoms, respectively. The locations of Cs⁺ ions and electrons are revealed by single-crystal X-ray diffraction and density functional theory studies. Highly selective Cs⁺ ion uptake is observed from an aqueous solution containing various alkali metal ions. Cs⁺ ions can be released from the crown-ether-like pores by the addition of aqueous chlorine as an oxidizing reagent. These results show that the POM capsule functions as an unprecedented “redox-active inorganic crown ether”, clearly distinguished from the non-redox-active organic counterpart.