A new mineral katiarsite, ideally KTiO(AsO₄), occurs in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with hatertite, bradaczekite, johillerite, yurmarinite, tilasite, arsmirandite, hematite, tenorite, As-bearing orthoclase, fluorophlogopite and aphthitalite. Katiarsite occurs as long prismatic to acicular, typically sword-like, crystals up to 3 μm×10 μm×50 μmin size, and rarely up to 0.15 mm long. Crystal forms are {011}, {201}, {100} and {001}. Katiarsite is transparent, colourless, with a vitreous lustre. The mineral is brittle. Cleavage was not observed, the fracture is uneven. Dcalc is 3.49 g cm-3. Katiarsite is optically biaxial (+), α = 1.784(3), β = 1.792(3), γ = 1.870(5); 2V_obs is small. Orientation is X=b, Y = a, Z = c. The Raman spectrum is reported. The chemical composition (wt.%, electron-microprobe data) is K₂O 18.98, Fe₂O₃ 5.07, TiO₂ 27.49, As₂O₅ 47.48, total 99.02. The empirical formula, calculated based on 5 O apfu, is K_1.00(Ti_0.85Fe_0.16³⁺ )Σ_1.01As_1.02O₅. The strongest reflections of the powder X-ray diffraction pattern [d,A(I )(hkl)] are 5.91(17)(110), 5.62(74)(011), 4.18(19)(202), 3.157(66) (013), 2.826(100)(221), 2.809(96)(022) and 2.704(19)(004). Katiarsite is orthorhombic, a = 13.174(4), b = 6.5635(10), c = 10.805(2) A, V = 934.3(3) A³, Z = 8, space group Pna2₁, by analogy with KTA, synthetic KTiO(AsO₄), a notable non-linear optical crystalline material. The name of the mineral reflects its chemical composition, kalium titanyl arsenate.