Exploring and developing new functional inorganic chalcogenides with unique structures is always one of the most important missions in solid-state chemistry, especially those with molecular structures. Herein, a novel quaternary thioarsenate, Cs 2 CdAsSe 5 , is found to be based on an unprecedented molecular (poly)chalcogenide cluster architecture, which has never been discovered in inorganic chalcogenide systems. This rare windmill-like [Cd 4 As 8 Se 16 (Se 2 ) 2 ] 8- cluster is made of four [CdSe 4 ] and [As(V)Se 4 ] tetrahedra via corner-sharing Se atoms and Se-Se bonds. Specifically, Cs 2 CdAsSe 5 exhibits a remarkable photocurrent response and a large computationally predicted birefringence, and the origin of the optoelectronic performance and optical anisotropy is confirmed by detailed theoretical investigation.