Garpenbergite is a new mineral (IMA2020-099) from the Garpenberg Norra mine, Hedemora, Dalarna, Sweden. It occurs with carlfrancisite and minor stibarsen, paradocrasite and filipstadite in a fractured skarn matrix of granular jacobsite, alleghanyite, kutnohorite and dolomite. Crystals are short-prismatic, up to 1.5 mm in length. They have a blackish to greyish brown colour, and are lustrous semi-opaque, with brown streak. Garpenbergite is brittle, with an uneven to subconchoidal fracture. Cleavage is distinct on {010}. Hardness ≈ 5 (Mohs) and VHN₁₀₀ = 650(40). D_calc = 4.47(1) g⋅cm⁻³ , overall n_calc = 1.85. Maximum specular reflectance values (%) obtained are 9.2 (470 nm), 9.1 (546 nm), 9.0 (589 nm) and 8.9 (650 nm). The empirical chemical formula of garpenbergite, based on electron microprobe data, is (Mn²⁺_3.97Mg_1.48Mn³⁺_0.26Zn_0.29)Σ_6.00(As_0.89Fe³⁺_0.04Mn³⁺_0.06Si_0.01)_Σ1.00(Sb_0.98Fe_0.02)_Σ1.00O₁₀[(OH)_1.99Cl_0.01]Σ_2.00. The five strongest Bragg peaks in the powder X-ray diffraction pattern [d, Å(I, %) (hkl)] are 3.05 (30) (002), 2.665 (100) (161), 2.616 (40) (301), 2.586 (25) (251) and 1.545 (45) (462). The orthorhombic unit-cell dimensions (in Å) are a = 8.6790(9), b = 18.9057(19) and c = 6.1066(6), with V = 1001.99(18) ų for Z = 4. The crystal structure was refined from single-crystal X-ray diffraction data in the space-group Ibmm to R1 = 3.7% for 957 reflections. Garpenbergite, ideally Mn₆As⁵⁺Sb⁵⁺O₁₀(OH)₂, is isostructural with manganostibite, Mn₇AsSbO₁₂, but possesses a cation vacancy (□) at an octahedrally coordinated structural site; the two minerals are thus related by the exchange Mn²⁺ + 2O^2– → □ + 2(OH)^– . The presence of hydroxyl groups is supported by vibration bands at 3647 and 3622 cm⁻¹ in the Raman spectrum of garpenbergite, and by bond-valence considerations.