The first alkaline-earth metal thioborate-thiophosphate Ba 3 (BS 3 )(PS 4 ) was designed from Ba 3 (BO 3 )(PO 4 ) by S-O substitution and fabricated experimentally. The [BS 3 ] pseudo-layers formed in the structure contribute to the strong optical anisotropy and a large birefringence of ∼0.11 at 1064 nm. The results enrich the structural and chemical diversity of chalcogenides.