We have characterized the crystal structure of sinnerite, Cu6As4S9, a rare sulfosalt mineral from the ores of the Lengenbach quarry, Binn Valley, Canton Valais, Switzerland, by single-crystal X-ray diffraction and chemical analysis. We found sinnerite to be structurally identical to synthetic Cu6As4S9. It is triclinic, space group P1, with cell parameters: a 9.103(2), b 9.860(3), c 9.111(2) Å, α 90.27(2), β 109.53(2), γ 107.58(2)°, V 729.6(4) Å3, and Z = 2. Semi-quantitative SEM-EDS analyses confirmed the Cu6As4S9 stoichiometry. The crystal structure of an untwinned crystal has been refined to R1 = 5.45%. It consists of a sphalerite substructure with 2/5 of the tetrahedra replaced by AsS3 pyramids; four pyramids form As4S12 clusters around the vacant anion positions of the sphalerite archetype. These pyramids are linked into twisted and branched chain-like structures with compositions As3S7 and As5S11. The chains are linked by CuS4 tetrahedra. Packing of these chains results in the OD (order-disorder) character of the sinnerite structure.