Mengeite, Ba(Mg,Mn2+)Mn3+4(PO4)4(OH)4·4H2O, is a new mineral from the Spring Creek copper mine, near Wilmington, South Australia, Australia, where it occurs as dark orange-red masses to 0.8 mm across in a quartz matrix. The streak is pale salmon pink and the luster is vitreous. Mengeite is non-fluorescent and the Mohs hardness is ∼3. The measured density is 3.40 g/cm3 and the calculated density is 3.43 g/cm3. Mengeite is biaxial (–) with α = 1.757(4), β = 1.776(4), γ = 1.781(4) measured in white light. Electron microprobe analyses results give the empirical formula, based on 24 O apfu: Ba1.09(Mg0.58Mn2+0.26Cu0.10Al0.02Na0.01Ca0.01□0.09)Σ1.07Mn3+3.88(PO4)4.07(OH)3.52·4.13H2O. The idealized formula is Ba(Mg,Mn2+)Mn3+4(PO4)4(OH)4·4H2O. Mengeite is triclinic, space group , with a = 5.4262(11), b = 5.4274(11), c = 16.387(3) Å, α = 87.61(3), β = 98.97(3), γ = 110.56(3)°, V = 446.28(16) Å3, and Z = 1. The crystal structure of mengeite was solved using synchrotron single-crystal X-ray diffraction data and refined to R1 = 0.0453 for 2115 observed reflections with F0 > 4σ(F0). It is based on [(M2(OH)2(AsO4)2] sheets that are linked in the c-direction alternately by [M3(H2O)4O2] octahedra and by BaO10 polyhedra.