Inorganic chalcogenides containing cations with lone-pair electrons have attracted considerable attention because of their potential applications in photocatalysis. In this research, two new copper thioarsenates with the lowest Cu-to-As ratio in the quaternary X/Cu/As/Q (X = inorganic cations; Q = chalcogen) system, namely Cs 3 CuAs 4 Q 8 (Q = S, Se), were obtained by a simple surfactant-thermal method at a low temperature. These two isostructural compounds belong to the monoclinic space group C 2/ c (no. 15) and are composed charge-balanced Cs + cations and two-dimensional anionic [CuAs 4 Q 8 ] 3- layers. Notably, photo-electrochemical measurements indicate that Cs 3 CuAs 4 Q 8 possesses a remarkable photocurrent response under simulated solar-light illumination. Further theoretical studies were performed to gain insights into the relationships between electronic structure and optical properties.