Inorganic chalcohalides have emerged as promising functional materials for a range of applications. However, most of them are chlorides and bromides, while iodides are still rare. Herein, a new quaternary iodine-based chalcohalide Ba₃GaS₄I has been acquired by the reaction of BaS, Ga, S and BaI₂ at 1173 K. Single-crystal XRD analysis reveals that Ba₃GaS₄I has a centrosymmetric structure (orthorhombic, space group Cmcm, a = 6.991(3) Å, b = 15.910(7) Å, c = 9.698(4) Å, V = 1078.6(7) ų, and Z = 4) and consists of isolated [GaS₄] tetrahedra with charge-balanced Ba²⁺ cations and I⁻ anions. Ba₃GaS₄I can be regarded as the product of chemical substitution of Ba₃GaS₄X (X = Cl, Br), which adopts a different space group, Pnma. The influences of the halogen elements on the bandgap and birefringence have been discussed through first principles calculations and structural comparison.