Leonardsenite (IMA2011-059), with ideal formula MgAlF5(H2O)2, is a new fumarole mineral from Eldfell volcano, Iceland. It has also been found in volcanic encrustations from the Hekla crater, Iceland. The mineral forms a soft and friable mass of white crystals up to 20 μm in length. The streak is white and the luster is earthy. The calculated density is 2.31 g cm−3. Leonardsenite is orthorhombic, space group Imma; the lattice parameters, obtained from Rietveld refinement of the XRPD data, are: a 7.055(1) Å, b 10.117(2) Å, c 6.813(1) Å, V 486.3(1) Å3, and Z = 4. The eight strongest reflections in the X-ray powder diffraction pattern are [d in Å (I) (hkl)]: 5.66 (100) (011), 4.92 (29) (101), 3.53 (27) (200), 3.03 (31) (031), 3.00 (38) (211), 2.30 (16) (231), 1.77 (19) (400), 1.76 (24) (242). Chemical analyses by energy-dispersive spectrometry using a scanning electron microscope produced a mean elemental composition as follows (wt.%): Mg 14.66, Al 16.16, F 52.98, O 15.88, H 1.78, total 101.46. The corresponding empirical formula, calculated on the basis of 2 cations pfu, is Mg1.00Al1.00[F4.64(OH)0.36]∑5.00(H2O)1.29. On the basis of chemical analyses and X-ray diffraction data, leonardsenite corresponds to the synthetic compound MgAlF5(H2O)2. The crystal structure of leonardsenite contains infinite chains of [AlF6] octahedra along the c-axis which are connected via common fluorine atoms to isolated [MgF4(H2O)2] octahedra. Leonardsenite is the first aluminum fluoride hydrated mineral of the inverse weberite group with general formula MIIMIIIF5(H2O)2. Leonardsenite belongs to the group of fluorides with interesting optical and catalytic properties. The name of this new species honors Erik Leonardsen (1934–), the former leader of the X-Ray Diffraction Laboratory of the Geological Institute, University of Copenhagen. The mineral and its name have been approved by the IMA-CNMNC (IMA2011-059).