Stibioústalečite, Cu₁₂ (Sb₂ Te₂)Se₁₃, was approved as a new mineral species from the Ústaleč mine, 15 km west of Horažďovice, SW Bohemia, Czech Republic. It occurs as metallic anhedral grains up to 0.1–0.3 mm in size, dark grey in color, in a calcite gangue. It is directly associated with hakite-(Hg), berzelianite, the not-yet approved phase Cu₁₂ (As₂ Te₂) Se₁₃ and uraninite. Stibioústalečite is brittle, with an indistinct cleavage and a conchoidal fracture; the calculated density is 5.676 g/cm³. In reflected light, stibioústalečite is isotropic, and grey in color; internal reflections were not observed. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 33.3; 546: 33.2; 589: 33.1; 650: 33.0. Electron microprobe analysis for holotype material (grain used for single-crystal X-ray study) gave (in wt. % – average of 7 spot analyses): Cu 34.10, Ag 1.22, Fe 0.04, Zn 0.09, Hg 0.33, Sb 9.39, As 0.70, Te 12.41, S 3.76, Se 37.59, total 99.63. On the basis of (As + Sb + Te) = 4 atoms per formula unit (apfu), the empirical formula of stibioústalečite is M(2₎ (Cu_5.75 Ag_0.25)_Σ6 M(1₎ (Cu_5.93 Hg_0.04 Zn_0.03 Fe_0.02)_Σ6.02 X(3₎ (Te_2.12 Sb_1.68 As_0.20)_Σ4 (Se_10.36 S_2.55)_Σ12.91. The ideal formula is Cu₆ Cu₆ (Sb₂ Te₂) Se₁₃, which requires Cu 33.33, Sb 10.64 Te 11.16 Se 44.87, total 100 wt. %. Stibioústalečite is cubic, I4̄ 3m, with unit-cell parameters a = 10.828(4) Å, V = 1269.6(9) ų, Z = 2. The crystal structure of stibioústalečite was studied by single--crystal X-ray diffraction data and it is isotypic with other members of the tetrahedrite group. The mineral is named after its type locality Ústaleč and its chemical composition, being the (Sb/Te) end-member in the possible ústalečite series.