Ruby-red crystals of Ag ₂Bi ₂S ₃Cl ₂ were synthesized from AgCl and Bi ₂S ₃ by cooling a melt from 770 K to room temperature. X-ray diffraction on powders and single-crystals revealed a triclinic crystal structure with special lattice constants (P 1̄ (No. 2), a=1085.0(2), b=717.2(1), c=1137.6(1)pm, α=89.80(1)°, β=74.80(1)°, γ=87.81(1)°). In the structure [Bi ^IIIS ₃Cl ₄] polyhedra form _∞ ²[BiS _3/2Cl _4/4] ^- double-layers by sharing common faces and edges. The silver(I) cations between the layers are coordinated either octahedrally by sulfide ions or tetrahedrally by sulfide and chloride ions. The deviations from the monoclinic space group P 1 2 ₁/c 1 are small and induce twinning along [010]. Further pseudosymmetry is based on the stacking of layer packages with the symmetry of the layer group P (2/c) 2 ₁/c 2/b.