Natroaphthitalite K(Na,K)₂Na(SO₄)₂, a new aphthitalite-group mineral, was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are anhydrite, langbeinite, hematite, tenorite, sanidine, johillerite, tilasite, and belomarinaite. Natroaphthitalite occurs as translucent yellow to colorless roundish single-crystal individuals up to 1 cm in diameter, sometimes combined in crusts up to 2 cm across, and as aqua-transparent colorless tabular to lamellar hexagonal crystals up to 2 mm across, sometimes skeletal, typically combined in parallel intergrowths or brush-like aggregates. D_calc is 2.664 g/cm³. Natroaphthitalite is optically uniaxial (þ), x ¼ 1.488(2), e ¼ 1.490(2). The chemical composition (wt.%, electron microprobe) is: Na₂O 22.54, K₂O 26.39, SO₃ 51.78, total 100.71. The empirical formula calculated based on 8 O apfu is K_1.735Na_2.252S_2.002O₈ (average of 15 spot analyses). The crystal structures of natroaphthitalite and the associated aphthitalite-group mineral belomarinaite, ideally KNaSO₄, from the Arsenatnaya fumarole were solved using single-crystal X-ray diffraction and refined to R₁ ¼ 0.0283 and 0.0261, respectively. The unit-cell parameters, Z, and space groups are a ¼ 5.6014(3), c ¼ 7.1507(5) Å, V ¼ 194.30(2) ų, Z ¼ 1, and P-3m1 for natroaphthitalite and a ¼ 5.6088(3), c ¼ 7.1858(4) Å, V ¼ 195.77(1) ų, Z ¼ 2, and P3m1 for belomarinaite. The crystal chemical formulae of natroaphthitalite and belomarinaite (Z ¼ 1) are ^XK_1.00 ^Y(Na_1.28K_0.72)_R2.00 ^MNa_1.00(^TSO₄)₂ and ^XK_1.00 ^Y1(Na_0.96K_0.04)_R1.00 ^Y2(K_0.94Na_0.06)_R1.00 ^MNa_1.00(^TSO₄)₂, respectively. Natroaphthitalite is isostructural with aphthitalite K₃Na(SO₄)₂ ¼ ^XK^YK₂ ^MNa(^TSO₄)₂ (both have centrosymmetric structures) with prevailing Na over K at the Y site of the crystal structure. Belomarinaite ^XK^Y2K^Y1Na^MNa(^TSO₄)₂ is the fully cation-ordered species, with acentric structure, in the series of natural aphthitalite-like K-Na sulfates.