New quaternary alkali-metal gallium selenites, AGa(SeO3)(2) (A = Li, Na, K, and Cs), have been synthesized through hydrothermal reactions. Single crystal X-ray diffraction has been used to determine the structures of the reported materials. The stoichiometrically equivalent materials crystallize in three-dimensional framework structures and share a similar bonding network that is composed of distorted GaO6 octahedra and asymmetric SeO3 polyhedra. However, all four materials exhibit different framework geometries attributable to the size of the alkali-metal cations and the orientation of lone pairs on Se4+ cations. Macroscopic centricities of the materials are also affected by interactions between the cation and the oxide ligands on GaO6 and SeO3 groups. Complete characterization including infrared spectroscopy, elemental analyses, thermal analyses, dipole moment calculations, and second harmonic generation properties for the compounds is also presented.