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K2S is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. K1+ is bonded to four equivalent S2- atoms to form a mixture of edge and corner-sharing KS4 tetrahedra. All K–S bond lengths are 3.23 Å. S2- is bonded in a body-centered cubic geometry to eight equivalent K1+ atoms.
K2S crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to five equivalent S2- atoms to form distorted KS5 trigonal bipyramids that share corners with twelve equivalent KS6 octahedra, corners with eight equivalent KS5 trigonal bipyramids, edges with six equivalent KS5 trigonal bipyramids, and faces with six equivalent KS6 octahedra. The corner-sharing octahedra tilt angles range from 31–60°. There are a spread of K–S bond distances ranging from 3.06–3.65 Å. In the second K1+ site, K1+ is bonded to six equivalent S2- atoms to form distorted KS6 octahedra that share corners with twelve equivalent KS6 octahedra, corners with twelve equivalent KS5 trigonal bipyramids, edges with six equivalent KS6 octahedra, faces with two equivalent KS6 octahedra, and faces with six equivalent KS5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 51°. There are four shorter (3.56 Å) and two longer (3.58 Å) K–S bond lengths. S2- is bonded in a 3-coordinate geometry to eleven K1+ atoms.