A series of alkali metal magnesium tungstates, A₄Mg(WO₄)₃ (A = Na, K), R₂Mg₂(WO₄)₃ (R = Rb, Cs), were synthesized from a high temperature solution, and their structures were determined by single-crystal X-ray diffraction. Interestingly, Na₄Mg(WO₄)₃ crystallizes in the monoclinic space group C2/c, while K₄Mg(WO₄)₃ having an identical stoichiometry with Na₄Mg(WO₄)₃, exhibits a different framework structure belonging to triclinic symmetry with the space group P. Isostructural Rb₂Mg₂(WO₄)₃ and Cs₂Mg₂(WO₄)₃ crystallize in the space group P2₁3 of cubic symmetry and reveal a three dimensional framework composed of isolated WO₄ tetrahedra, MgO₆ octahedra and RO₁₂ (R = Rb, Cs) polyhedra. The effect of the alkali metal cation size on the framework structures of magnesium tungstate has been discussed in detail. In addition, the infrared spectra, as well as the UV-Vis-NIR diffuse reflectance spectroscopy data, are reported. The first-principles theoretical studies are also carried out to aid the understanding of electronic structures and linear optical properties.