Complexes made by hosts that completely surround their guests provide a means to stabilize reactive chemical intermediates, transfer biologically active cargo to a diseased cell, and construct molecular‐scale devices. By the virtue of inorganic host–guest self‐assembly, nucleation processes in the cavity of a {P₈W₄₈}‐archetype phosphotungstate has afforded a nanoscale 16‐AlIII‐32‐oxo cluster and its GaIII analogue that contain the largest number of A^lIII/Ga^III ions yet found in polyoxometalate (POM) chemistry. Interestingly, the rich Lewis acid AlIII centers within the Lewis base POM support shows an exceptional proton conductivity of 4.5×10⁻² S cm⁻¹ (85 °C, 70 % RH; RH: relative humidity), which is by far the highest conductivity reported among POM‐based single‐crystal proton conductors.