Three specimens of galkhaite from the Getchell mine (G), Nevada, USA; the Gal-Khaya As-Hg-Sb deposit (Gk), Yakutia, Russia; and Signols (S), Piedmont, Italy, have been fully characterized through single-crystal X-ray diffraction and electron-microprobe analyses. Chemical data indicate the formulae [(Hg4.61Zn0.38Fe0.04)Σ5.03(Cu1.03Ag0.04)Σ1.07]Σ6.10(Cs0.76Tl0.20)Σ0.96(As3.81Sb0.04)Σ3.85S12.06 (G), [(Hg4.66Zn0.32)Σ4.98(Cu0.55Ag0.45)Σ1.00]Σ5.98(Cs0.74Tl0.06)Σ0.80(As3.42Sb0.56)Σ3.98S12.05 (Gk), and [(Hg4.10Zn0.85)Σ4.95(Cu0.89Ag0.16)Σ1.05]Σ6.00Cs0.95(As3.64Sb0.16)Σ3.80S12.19 (S). Unit-cell parameters are a 10.405(3) Å (G), 10.443(1) Å (Gk), and 10.332(2) Å (S), space group I3m. The crystal structure of the three specimens has been solved to R1 = 0.029 (G), 0.028 (Gk), and 0.031 (S). It is a three-dimensional framework of HgS4 and AsS3 polyhedra, with Cs hosted in large 12-fold coordinated cavities. The introduction of Cs+, as well as minor Tl+, at these structural positions requires the replacement of Hg2+ by Cu+. The mixed site occupancy at the Hg site of galkhaite is a case of valency-imposed double site-occupancy. Consequently, the idealized chemical formula of galkhaite can be written as (Hg5Cu)CsAs4S12. Taking into account the possible existence of vacancy in the structural cavities, the formula can be generalized as (Hg5+xCu1–x)Σ6Cs1–xAs4S12 (x ≈ 0).