The crystal structure of tooeleite, Fe6(AsO3)4(SO4)(OH)4·4H2O, has been solved from high-resolution synchrotron XRD powder data recorded on a sample from Tooele county, Utah. The structure is monoclinic, space group C2/m, a = 8.9575(1), b = 6.4238(1), c = 9.7912(1) Å, β = 96.032(1)°, V = 560.27(3) Å3, dcalc = 3.16 g/cm3. The structure was solved by direct methods and atomic positions, site occupancies, and isotropic displacement parameters were refined by the Rietveld method. The AsO3 pyramids bond to FeO6 octahedra by both edge- and corner-linkage, forming layers that intercalate SO4 groups. Assignment of structural H2O and OH groups were done from bond-valence analysis. Tooeleite is the only arsenite-sulfate mineral known and has been recently identified as the main constituent of stromatolite-like deposits in the Carnoulès acid mine, Gard, France.