The new mineral genplesite, Ca₃Sn(SO₄)₂(OH)₆·3H₂O, the first natural tin sulfate, is found in cavities in massive chalcopyrite ore at the Oktyabr’skoe Cu-Ni-Pd-Pt deposit (Oktyabr’sky mine), Talnakh, Norilsk district, Siberia, Russia. It is a latestage hydrothermal mineral associated with greenalite, chamosite, pectolite, ferroactinolite, calcite and fluorapatite. Genplesite occurs as equant, short prismatic or thick tabular hexagonal crystals up to 0.5 mm and crystal clusters up to 0.6 × 1.2 mm. The major crystal forms are {1 0 0} and {0 0 1} whereas {1 0 2} and {1 0 1} are minor. Genplesite is transparent, colourless, with vitreous lustre. It is brittle, the Mohs hardness is ca. 3. Cleavage is not observed, the fracture is conchoidal. D_meas= 2.78(1), D_calc= 2.773 g · cm^–3. Genplesite is optically uniaxial (-), ω = 1.597(2), ε = 1.572(2). The IR spectrum is reported. The chemical composition (wt%, electron microprobe data, H₂O calculated for 3 molecules per formula unit, pfu) is: CaO 28.67, Al₂O₃0.11, GeO₂0.50, SnO₂24.20, SO₃27.25, H₂O_calc18.34, total 99.07. The empirical formula calculated on the basis of 17 O apfu is: Ca_3.01(Sn_0.95Ge_0.03Al_0.01)_Σ0.99S_2.01O₈(H)₆· 3H₂O. Genplesite is hexagonal, P 63/mmc, a = 8.5139(2),c = 11.1408(3) Å, V = 699.37 (1) ųand Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I)(hkl)] are: 7.38(68)(1 0 0), 4.259(46) (1 1 0), 3.503(15)(2 0 1), 3.383(1 0 0)(1 1 2), 2.616(13)(2 0 3), 2.493(14)(2 1 2), 2.249(14)(3 0 2) and 2.130(17)(1 0 5, 2 2 0). Genplesite is a member of the fleischerite group. Its crystal structure was solved from single-crystal X-ray diffraction data (R = 0.016). The major building unit of the structure is a column consisting of isolated Sn⁴⁺(OH)₆octahedra and trimers of edgesharing CaO₂(OH)₄(H₂O)₂polyhedra. Adjacent columns are interconnected via H₂O molecules and SO₄tetrahedra. Genplesite is named in honour of Gennadiy N. Plesin (b. 1963), a mine surveyor at the Oktyabr’sky mine and an amateur mineralogist who found the mineral.