Four open-framework Ag–Sn–S compounds K₂Ag₂Sn₂S₆ (1); K₂Ag₂SnS₄ (2); Rb₂Ag₂SnS₄ (3); and Cs₂Ag₂SnS₄ (4) have been synthesized using a solvothermal method. Compound 1 possesses a unique three-dimensional (3D) structure in which Ag⁺ ions are two-coordinated. Compounds 2–4 have the same layered structure in which Ag⁺ ions are tetrahedrally coordinated. Photocatalytic degradation properties of methylene blue have been investigated and compound 1 displays excellent photodegradation activities. The photoelectric response properties, optical properties, and theoretical calculations of these compounds have also been studied.