The new mineral fehrite (IMA 2018-125a), MgCu4(SO4)2(OH)6 · 6H2O, is a member of the ktenasite group and the Mg-analogue of ktenasite, ZnCu4(SO4)2(OH)6 · 6H2O. The mineral was found in the Casualidad mine near Baños de Alhamilla, Almeria, Spain, in association with clinoatacamite, kapellasite, gordaite, serpierite, connellite and gypsum. The transparent turquoise-coloured mineral has a vitreous lustre, exhibits a pale blue-green streak and shows distinct pleochroism. It forms radial aggregates of thin lath-like crystals of up to 200 μm in length. Fehrite is not fluorescent. The monoclinic crystals show a perfect cleavage parallel to {001}. The mineral has a brittle tenacity and an uneven fracture. The calculated density is 2.73 g/cm3, the calculated mean refractive index is 1.584. The strongest lines observed in the X-ray powder diffraction pattern are [d in Å/Irel in %/(hkl)] 11.94/100/002, 5.92/31/004, 2.66/12/202, 4.85/11/013, 3.93/11/006 and 2.96/10/008. The chemical composition, measured by means of an electron probe micro-analyser, was determined at (wt.%): MgO 5.31, MnO 0.49, CuO 33.12, ZnO 11.48, SO3 26.01, H2Ocalc. 24.63, total 101.04. The empirical formula based on 20 O pfu., including 6(OH) and 6(H2O), is Mg0.87Cu2.74Zn0.93Mn0.05S2.14O8(OH)6 · 6H2O. The simplified end member formula is MgCu4(SO4)2(OH)6 · 6H2O which requires MgO 5.92, CuO 46.74, SO3 23.52, H2O 23.82, total 100.00 (wt.%). Fehrite is monoclinic with space group P21/c (#14). Unit cell parameters determined by X-ray single crystal diffraction are a = 5.6062(8), b = 6.1294(11), c = 23.834(3) Å, β = 95.29(1)º, V = 815.5(2) Å3, Z = 2. The mineral is isotypic with ktenasite with Mg in place of Zn. The name is for the late Karl Thomas Fehr (1954 – 2014), Professor of Mineralogy at the Department of Geo- and Environmental Sciences at the Ludwig-Maximilians-University Munich, Germany.