Ammoniomagnesiovoltaite (IMA no. 2009-040) was found on a burning coal dump in an abandoned open coal pit at Pécs-Vasas, Mecsek Mountains, South Hungary. It is closely associated with tschermigite, sabieite, kieserite, pickeringite, and hexahydrite, but other ammonium minerals (salammoniac, mascagnite, clairite, boussingaultite, efremovite, koktaite, mohrite, ammoniojarosite, godovikovite, letovicite) as well as native sulfur, millosevichite, alunogen, metavoltine, voltaite, gypsum, anhydrite, halotrichite, butlerite, jarosite, and copiapite also occur in the surrounding area. Ammoniomagnesiovoltaite was formed by the interaction of organic matter with oxidizing iron sulfides (pyrite and marcasite). It forms crusts consisting of cubo-octahedral and octahedral crystals up to 0.05 mm in size. The mineral is translucent and pale yellow to yellowish brown in color, with white streak and vitreous luster. Its Mohs hardness is about 2–3 and the measured density is 2.55(1) g/cm3. Cleavage was not observed; fracture is uneven. Optically it is isotropic with n = 1.60(1). The chemical composition was found to be: SO3 47.56, Al2O3 3.31, Fe2O3 12.28, FeO 5.94, MnO 1.18, MgO 7.v02, K2O 0.05, (NH4)2O 3.11, and H2O (calc.) 17.72; total 98.17 wt.%. The empirical formula is [(NH4)2.18 K0.02]∑=2.20 (Mg3.19 Fe2+1.51 Mn0.30)∑=5.00 (Fe3+2.81 Al1.19)∑=4.00 S10.87 O44.70·18H2O, and its simplified formula is (NH4)2Mg5Fe3+3Al(SO4)12·18H2O. The strongest six lines in the X-ray powder diffraction pattern are [dhkl in Å (Iobs %, hkl)]: 5.59 (100, 422), 3.420 (72, 800), 3.562 (66, 731), 1.7836 (25, 15.3.1), 1.5582 (25, 16.6.4), and 6.85 (24, 400). According to X-ray powder diffraction, ammoniomagnesiovoltaite is structurally analogous to voltaite-group minerals; it is cubic, space group Fd3̅c, a = 27.260(2) Å, V = 20257(2) Å3, Z = 16. The new mineral is named for the dominant ammonium and magnesium in the composition and its membership in the voltaite group.