The crystal structure of the rare mineral fettelite was solved using intensity data collected from a twinned crystal from Chañarcillo, Copiapó Province, Chile. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is monoclinic (space group C2) with a = 26.0388(10), b = 15.0651(8), c = 15.5361(8) Å, β = 90.48(1)°, and V = 6094.2(5) Å3. The refinement of an anisotropic model led to an R index of 0.0656 for 7143 observed reflections [I > 2σ(I)] and 0.0759 for all 17 447 independent reflections. Fettelite is intimately twinned with six twin domains. The structure consists of the stacking of two module layers along : an A module layer with composition [Ag6As2S7]2− and a B module layer with composition [Ag10HgAs2S8]2+. The As atoms form isolated AsS3 pyramids typical of sulfosalts, Hg links two sulfur atoms in linear coordination, and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The A module layer found for fettelite is identical to that described for the minerals belonging to the pearceite-polybasite group. On the basis of information gained from this characterization the crystal chemical formula was revised according to the structural results, yielding [Ag6As2S7][Ag10HgAs2S8] (Z = 8).