AbstractTernary metal dichalcogenides alloys exhibit compositionally tunable optical properties and electronic structure and therefore, band gap engineering by controllable doping would provide a powerful approach to promote their physical and chemical properties. Herein we obtained ternary SnS_2−xSe_x alloys with tunable chemical compositions and optical properties via a simple one-step solvothermal process. Raman scattering and UV-vis-NIR absorption spectra reveal the composition-related optical features and the band gaps can be discretely modulated from 2.23 to 1.29 eV with the increase of Se content. The variation tendency of band gap was also confirmed by first-principles calculations. The change of composition results in the difference of crystal structure as well as morphology for SnS_2−xSe_x solid solution, namely, nanosheets assemblies or nanosheet. The photoelectrochemical measurements indicate that the performance of ternary SnS_2−xSe_x alloys depends on their band structures and morphology characteristics. Furthermore, SnS_2−xSe_x photodetectors present high photoresponsivity with a maximum of 35 mA W⁻¹ and good light stability in a wide range of spectral response from ultraviolet to visible light, which renders them promising candidates for a variety of optoelectronic applications.