Wilhelmkleinite, a new zinc iron hydroxy-arsenate, ideally ZnFe (AsO)(OH), is monoclinic, space group P2/n, with a = 6.631(1) Å, b = 7.611(1) Å, c = 7.377(1) Å, β = 91.80(1), Z = 2, from single crystal studies. The strongest reflections in the X-ray powder diffraction data are (d,l,hkl) 3.385(100)(021), 3.315(78)(200/012), 2.939(47)(112). The mineral occurs at the Tsumeb mine, Namibia. It forms twinned spearhead-shaped crystals up to 5 mm in length. The blackish green translucent crystals which show strong pleochroism from olive-green to emerald-green and reddish-brown are found on Al-rich scorodite. They show no fluorescence, their streak is green and the hardness on Mohs'scale is 4.5. The refractive index is about 1.94, the calculated density results in 4.364g/cm. Electron microprobe analyses yielded ZnO 13.81, FeO 33.60 and AsO 46.69.