new quaternary metal thiophosphate, Cs₂Ga₃PS₈, in triclinic P  space group has been successfully synthesized by a reactive-flux method. Its structural framework is derived from well-known AM^IIIM^IVQ₄ (A = alkali metal; M^III = Al, Ga, In; M^IV = Si, Ge, Sn; Q = S, Se) system and composed of two-dimensional 2 ∞[Ga₃PS₈]^2– layers separated by Cs⁺. The compound exhibits a wide band gap of 3.08 eV and congruent-melting behavior with melt point of 645 ℃. Interestingly, Cs₂Ga₃PS₈ exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm. Moreover, electronic structure calculations indicate that Cs₂Ga₃PS₈ is a direct band gap compound and its luminescent process can be mainly ascribed to electron transfer from the S-3p and Ga-4p states to S-3p and P-3p.