We synthesized a new hydrous silicate in the pressure range from 20 GPa to 24 GPa at 800-1300°C. This phase, named tentatively as phase G, has a hexagonal unit cell, a=4.790(3)Å and c=4.344(3)Å, and V=86.3(2)Å3 and the atomic ratio Mg/Si=0.66±0.03. SIMS analysis revealed that it contains 14.5±2.0wt% water. Phase G has a chemical formula of Mg1.14Si1.73H2.81O6 and a density of 3.37g/cm3. Phase G coexists with periclase and Mg-perovskite under the lower mantle conditions, and thus it can be a reservoir of water in cold slabs penetrating into the lower mantle.