Trisodium rubidium heptamagnesium hexakis(orthophosphate) exhibits a new structure type, with MgOx (x = 5 and 6) polyhedra linked directly to each other through common corners or edges and reinforced by corner-sharing with PO4 tetrahedra. The resulting anionic three-dimensional framework leads to the formation of channels in which the Na+ cations are located, while the Rb+ cations are located in large interstitial cavities.