The new cancrinite-group mineral steudelite, ideally (Na₃☐)[(K,Na)₁₇Ca₇]Ca₄(Al₂₄Si₂₄O₉₆)(SO₃)₆F₆·4H₂O, was discovered in syenite ejectum from the Sacrofano paleovolcano, Latium region, Italy. The associated minerals are sanidine, diopside, andradite, biotite, leucite, haüyne, sacrofanite, biachellaite, liottite, and secondary dioctahedral smectite. Steudelite forms colourless, thick-tabular, isometric and short-prismatic crystals up to 7 mm across. Cleavage is distinct on {10 1 ¯ 0}; perfect parting on {0001} is observed. Steudelite is brittle, with uneven fracture. The Mohs’ hardness is 5. Measured and calculated density are equal to 2.51 (1) and 2.511 g cm^–3, respectively. The IR spectrum shows the presence of H₂O molecules, SO₃^2– and minor SO₄^2– anionic groups. The chemical composition of steudelite is (wt%; electron microprobe, H₂O determined by the modified Penfield method; total sulfur apportioned between SO₂ and SO₃ based on structural data): Na₂O 7.40, K₂O, 8.42, CaO 3.54, Al₂O₃ 26.46, Fe₂O₃ 0.18, SiO₂ 30.96, SO₂ 4.74, SO₃ 5.18, F 1.66, Cl 0.84, H₂O 1.6, –O≡Cl,F − 0.89, total 100.09. The empirical formula is H_8.22Na_11.06K_8.28Ca_11.18(Al_24.04Fe³⁺_0.10Si_23.86O₉₆)(SO₃)_3.43(SO₄)_3.00F_4.04Cl_1.10O_3.89. The crystal structure of steudelite is based on the afghanite-type aluminosilicate framework containing a column of cancrinite cages and isolated cancrinite and liottite cages whose populations are {Na_3.14Ca_0.14☐_0.72[(H₂O,OH)_3.72Cl_0.28]}, {Ca₄F_4.02Cl_0.60}, and {K_8.40Na_8.13Ca_7.10(SO₃)_3.24(SO₄)_2.76}, respectively. The SO₃^2– and SO₄^2– groups occur in the liottite cage, at two sites with the occupancies ^S1[(SO₄)_0.69(SO₃)_0.31] and ^S2[SO₃]₂. Steudelite is hexagonal, space group P-62c, with a = 12.89529 (15), c = 21.2778 (3) Å, V = 3064.21 (8) ų, and Z = 1. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 11.15 (28) (100), 4.799 (25) (104), 3.973 (16) (105), 3.721 (47) (300), 3.305 (100) (214, 303, 400), 2.661 (17) (008), 2.149 (21) (330). The mineral is named in honour of the outstanding German chemist, a specialist in chemistry of sulfur Prof. Ralf Steudel (1937–2021).