Sr5Ga8O3S14, which is a novel nonlinear optical oxysulfide, was discovered via solid-state reaction. The compound crystallizes in noncentrosymmetric space group P2(1)2(1)2 (No. 18) of the orthorhombic system. The crystal structure is derived from the melilite structure and composed of anionic (2)(infinity)[Ga8O3S14](10-) layers intercalated by Sr2+ cations. The anionic layers are constructed by [GaOS3](5-) and [GaS4](5- )tetrahedra via either corner sharing or edge sharing, while typical melilite only features edge-sharing mode. Because of the existence of electronegative O atoms, it possesses a large band gap (3.9 eV) with a wide transparent region of 0.3-13.4 mu m. It exhibits second-harmonic-generation response, which is 0.8 times that of the reference AgGaS2 (AGS) in the particle size range of 20-41 mu m under laser irradiation at a wavelength of 2.09 mu m laser. Density functional theory calculations indicate that the band edges are mainly composed of the Ga-S bond orbitals, with little participation of O 2p near the valence band maximum.