Sb ₂S ₃ and CuCl were reacted in the ionic liquid [BMIm]Cl ⋅ 4.4AlCl ₃ (BMIm=1-n-butyl-3-methylimidazolium) at 200 °C. Upon cooling to room temperature, orange-red colored, air-sensitive crystals of Cu(Sb ₂S ₃)[AlCl ₄] precipitated. X-ray diffraction on a single-crystal revealed an orthorhombic crystal structure, in which cationic [Cu(Sb ₂S ₃)] ⁺ layers are separated by tetrahedral [AlCl ₄] ⁻ anions. The uncharged Sb ₂S ₃ partial structure consists of one-dimensional strands with covalent Sb−S single bonds. The copper(I) cation is coordinated by three sulfur atoms and by one of chlorine atoms of the [AlCl ₄] ^– anion. An optical band gap of 2.14 eV was deduced from UV/Vis spectra. In very good agreement, a band gap of 2.07 eV results from DFT-based calculations involving a new implementation of the bifunctional formalism for the exchange energy. By treatment with 0.1 molar hydrochloric acid, AlCl ₃ was leached from Cu(Sb ₂S ₃)[AlCl ₄] yielding a compound with the presumed composition Cu(Sb ₂S ₃)Cl. Hydrolysis at higher pH resulted in Cu _2–xS and Sb ₄O ₅Cl ₂.