The new mineral zanelliite, ideally PbCu9[AsO3.5(OH)0.5]2(AsO4)2(OH)9(H2O)3, was discovered at the Grosses Chalttal deposit, Mürtschenalp district, Glarus, Switzerland. It occurs as aggregates of {100} platy crystals, up to 0.05 mm in size, turquoise-green in colour. Lustre is vitreous. The calculated density is 4.425 g cm–3. The empirical formula, based on 28 O atoms per formula unit, is (Pb0.90Ca0.08)Σ0.98(Cu9.00Zn0.01)Σ9.01(As3.84Mo0.17)Σ4.01O15.17(OH)9.83(H2O)3. The occurrence of H2O was confirmed through micro-Raman spectroscopy and structural analysis. Zanelliite is monoclinic, space group C2/c, with unit-cell parameters a = 39.395(7), b = 5.5563(9), c = 10.5373(17) Å, β = 95.109(6)°, V = 2297.3(7) Å3 and Z = 4. The eight strongest reflections in the observed powder X-ray diffraction pattern are [d in Å (I)]: 19.7 (100), 4.86 (74), 4.48 (31), 4.28 (40), 3.777 (42), 2.964 (82), 2.708 (74), and 2.569 (37). The crystal structure of zanelliite was refined to R1 = 0.1215 for 2077 unique reflections with F > 4σ(F) and 125 refined parameters. Two different kinds of layers can be identified, i.e. a Cu–Pb–As heteropolyhedral layer, with composition [PbCu6As4O14(OH)6(H2O)], and a Cu layer, with formula [Cu3O(OH)4(H2O)2]. The crystal structure is composed of an alternating stacking of these two types of layers in a 1:1 ratio. Zanelliite is a supergene mineral whose origin is related to the oxidation of primary Cu ores. It is associated with brochantite, chrysocolla, heimite and parnauite. A second occurrence of zanelliite was reported from the Hochmättli deposit, Mürtschenalp district, Glarus, Switzerland. Zanelliite honours the mineral collector, mathematics and geography teacher Dr. Remo Zanelli (b. 1977) for his contributions to the knowledge of the secondary minerals of the ore deposits of the Glarus Alps. The mineral, its name, and its symbol (Znl) were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2024-061).