Leogangite, ideally Cu₁₀(AsO₄)₄(SO₄)(OH)₆·8H₂O, was found on sample material from mine dumps of the Danielstollen and Inschlagalm in the Schwarzleo valley, both part of the polymetallic ore district of Leogang, Salzburg province, Austria. It is intergrown with olivenite and malachite, all together coating small voids within a tennantite-bearing dolomite breccia, and is assumed to have formed as an alteration product of arsenic-rich fahlore. Associated minerals are tyrolite, parnauite, strashimirite, euchroite, brochantite, langite, posnjakite, and devilline. The new mineral crystallizes with monoclinic symmetry, space group C2/c (no. 15), with a=21.770(7)20[`4]{\it 20}{\bar {\it 4}} ); 5.44, 50, (400); 3.625, 50, (600); 3.090, 40, (023). The structure of the mineral was refined to R1=0.095 for 1768 unique reflections with $F^2_{\rm o} > 4\sigma(F^2_{\rm o})$F^2_{\rm o} > 4\sigma(F^2_{\rm o}) and is a new representative of the rare [5]-coordinated Cu²⁺ compounds with edge-sharing CuO₅ square pyramids. The main motif consists of [Cu₄O₁₄] tetramers. Together with the AsO₄ tetrahedra and the remaining CuO₅ polyhedron they build complex heteropolyhedral layers parallel to (100), which are linked by the SO₄ tetrahedron and parts of the hydrogen bonds along the a-axis, thus forming a complex open framework structure. Chemical, optical and X-ray powder pattern characteristics are the distinguishing features to similar minerals like parnauite, tyrolite, or clinotyrolite.