The wheel‐shaped, potassium‐templated 27‐tungsto‐3‐arsenate(III) K⊂{(β‐AsIIIW8O30)(WO(H2O))}314– (1) was synthesized by simple one‐pot condensation of the trilacunary B‐α‐AsIIIW9O339– precursor in aqueous, acidic KCl solution. Polyanion 1 comprises three β‐{AsW8O30} units linked via three trans‐{WO(H2O)} groups, forming a cyclic assembly with a potassium ion located in the central cavity. The rubidium‐analogue Rb⊂{(β‐AsIIIW8O30)(WO(H2O))}314– (2) could also be prepared, by addition of a small amount of Rb+ ions to a solution containing a large excess of K+ ions. This indicates that the rigid {(β‐AsIIIW8O30)(WO(H2O))}315– (As3W27) host exhibits high selectivity for Rb+ ion guests compared to K+ ions. Polyanions 1 and 2 were characterized in the solid state by single‐crystal XRD, FT‐IR, TGA, and elemental analysis. We have prepared a trimeric, wheel‐shaped 27‐tungsto‐3‐arsenate(III) host, M⊂As3W27 (M = Rb, K), which exhibits high selectivity for Rb+ ions, in analogy to organic crown ethers.