A new cobalt-indium silicate, Rb6(InCo)2(Si 9O26), has been synthesized via a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction. It crystallizes in the noncentrosymmetric orthorhombic space group Aba2. (No. 41) with a = 20.779(1) Å, b = 12.0944(6) Å, c = 10.7761(5) Å, V = 2708.1(2) Å3, and Z = 4. The structure consists of 20-membered-ring silicate single layers of corner-sharing SiO 4 tetrahedra interconnected by dimers of edge-sharing CoO4 tetrahedra and InO6 octahedra into a 3D framework. The Si:O ratio for the title compound is the lowest among the known single-layer silicates. Magnetic susceptibility confirms the divalent state of the cobalt ion. The powder sample has a second-harmonic-generation signal, confirming the absence of a center of symmetry in the structure.