The new mineral zincomenite, β-ZnSeO₃, is found in active fumaroles belonging to the Northern fumarole field at the First scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with sofiite, sellaite, fluorite, halite, anhydrite, cotunnite, chubarovite, flinteite, etc. Zincomenite occurs as tabular, equant or prismatic crystals up to 0.2 mm sometimes combined in clusters up to 0.3 mm across, or interrupted incrustations up to 0.7 × 1 cm overgrowing basalt scoria. Partial to complete pseudomorphs of zincomenite after sofiite are typical. The crystal forms are {101}, {010}, {100} and {013}. T-shaped twins with (012) as twin plane were found. Zincomenite is transparent, colourless, white or pale beige with adamantine lustre. Cleavage and parting were not observed and the fracture is uneven. Dcalc is 4.760 g cm^-3. Zincomenite is optically biaxial (-), a = 1.744(5), β = 1.860(5), γ = 1.875(5). The IR spectrum is reported. The chemical composition (wt.%, electron microprobe data) is: ZnO 42.53, SeO₂ 56.67, total 99.20. The empirical formula calculated on the basis of 3 O apfu is: Zn_1.02Se0.99O₃. Zincomenite is orthorhombic, Pbca, a = 7.1971(2), b = 6.2320(2), γ = 11.9914(3) Å, V = 537.84(2) Å3 and Z = 8. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 4.612(26)(102), 3.601(77)(200), 3.119(48)(210), 3.048 (38)(113), 3.014(100)(211, 021), 2.996(56)(004), 2.771(19)(123, 104, 212), 2.459(23)(213, 023), 2.311(20)(123, 221, 204), and 2.162 (19)(214, 024). Zincomenite is a representative of the CuSeO3 structure type. Its crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0188), contains layers formed by Zn₂O₈ dimers (consisting of edge-sharing ZnO5 trigonal bipyramids; each dimer shares four vertices with the neighbouring ones) linked via (SeO₃)^2- groups (with Se⁴⁺in trigonal pyramidal coordination) to form an open framework. The mineral is named in allusion to its chemical composition: zinc selenite (the Greek μ γγaγ means moon, indicating selenium).