A series of alkali metal magnesium tungstates, A(4)Mg(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 (1) over bar. Isostructural Rb2Mg2(WO4)(3) and Cs2Mg2(WO4)(3) crystallize in the space group P2(1)3 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.