Avdoninite, a new mineral species, has been found together with euchlorite, paratacamite, atacamite, belloite, and langbeinite hosted in exhalation sediments of the Yadovitaya fumarole in the Second Cinder Cone at the Northern Breach of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. Avdoninite occurs as imperfect, short prismatic and thick tabular crystals up to 0.2 mm long, with (001) and (100) forms, crystal aggregates, and pseudomorphs (together with atacamite) after melanothallite observed. The new mineral is brittle, with the Mohs hardness 3 (for aggregates). Density is 3.03 g/cm³ (meas.) and 3.066 g/cm³ (calc.). Avdoninite is biaxial and optically neutral, with α = 1.669, β = 1.688, γ = 1.707, 2V = −90°. Dispersion is not observed. Optical orientation: Y = c, X = b? Pleochroism is absent. The infrared spectrum suggests the presence of water molecules in avdoninite. Electron microprobe chemical analysis has given (wt %) K₂O 11.94 (±0.4), CuO 51.43 (±0.7), Cl 37.07 (±0.6), H₂O (determined by the Penfield method) 6.9, −O=Cl₂ −8.37, total 98.97. The empirical formula is K_1.96Cu_5.00Cl_8.09(OH)_3.87. · 1.03H₂O. Avdoninite is monoclinic, space group P2/m, P2, or Pm; a = 24.34(2) Å, b = 5.878(4) Å, c = 11.626(5) Å, β = 93.3(1)°, V = 1660.6(20) ų, Z = 4. The compatibility index is good: 1 − K _p/K _c = 0.056 for D _calc and 0.044 for D _meas. The strongest lines in the X-ray powder diffraction pattern (d, Å (I, %) (hkl)) are 11.63(100)(001), 5.88(20)(010), 5.80(27)(002), 5.73(17)( [`1]\overline 1 02), 2.518(19)(21 [`4]\overline 4 ), 2.321(17)(005). Avdoninite is identical to a technogenic analogue previously described from the Blyava volcanic-hosted massive sulfide deposit, Orenburg oblast, Russia. The new mineral is named after Vladimir Nikolaevich Avdonin (born 1925), a senior researcher of the Ural Geological Museum of the Ural State Mining University. The type material of avdoninite from Kamchatka is deposited in the Mineralogical Museum of the Department of Mineralogy, St. Petersburg State University, St. Petersburg, Russia. The registration number is 19175.