Cs22In6(SiO4)4 was synthesized by the reaction of appropriate starting materials at 673 K, followed by slow cooling to room temperature, in arc-welded tantalum ampoules. According to single-crystal X-ray analysis, the compound crystallizes in a new structure type (P21/n(no. 14), a = 14.3533(4), b = 16.1712(4), c = 25.0135(7) Å, β = 94.368(1), Z = 4), consisting of [In6]6– clusters with the shape of a distorted octahedron or more appropriately described as a condensate of three face sharing tetrahedra. The cluster is the first example of a “hypoelectronic” isolated [In6]6– indium cluster. The oxosilicate indide can be regarded as a “double salt“, Cs6In6 on one hand and the oxosilicate Cs4SiO4 ( × 4) on the other, which form the quaternary structure by inhomogeneous intergrowth of partial structures. The electronic structure of Cs22In6(SiO4)4 was examined by DFT calculations and compared to the one of Rb2In3, which exhibits linked In6 polyhedra. According to the DOS the title compound is a semiconductor with a band gap of 0.5 eV, which is in agreement with its observed insulating character. [In6]6– is an “isolated” cluster bearing inert electron pairs at each vertex. In contrast, [In6]4– in Rb2In3 only exhibits inert pairs at the apical atoms. The four basal atoms are linked to neighboring clusters by covalent bonds forming a 2D network. These bonding scenarios are supported by the analysis of the projected density of states, the electron localization function and the partitioning of the electron density according to Bader.