A novel indium silicate, Rb3In(H2O)Si 5O13, has been synthesized using a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction. The structure consists of five-membered rings of corner-sharing SiO4 tetrahedra connected via corner sharing to four adjacent five-membered rings to form a 3D silicate framework that belongs to the CdSO4 topological type. The InO6 octahedron shares five of its corners with five different SiO4 tetrahedra to form a 3D framework that delimits two types of channels to accommodate the rubidium cations. The sixth corner of InO6 is coordinated H2O. The structure is related to that of the titanium silicate ETS-10, and these are the only two metal silicates that have the CdSO4-topological-type structure. In addition, the crystal of Rb3In(H2O)-Si 5O13 shows an intense second harmonic generation signal. Crystal data: H2Rb3InSi5O3 14, monoclinic, space group Cc (No. 9), a = 9.0697(5) Å, b = 11.5456(6) Å, c = 13.9266(8) Å, β= 102.300(1)°, V= 1424.8(1) Å3, and Z = 4.