A new Al-bearing hydrous Mg-silicate that we named as 23 Å phase was synthesized at 10 GPa and 1000 °C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg7Si2O8(OH)6] + Al2O3 + H2O. The chemical composition of this new 23 Å phase is Mg11Al2Si4O16(OH)12, and it contains about 12.1 wt% water. Powder X-ray diffraction and electron diffraction patterns show that this new 23 Å phase has a hexagonal structure, with a = 5.1972(2), c = 22.991(4) Å, and V = 537.8(2) Å3, and the possible space group is P6̄c2, P63cm, or P63/mcm. The calculated density is 2.761 g/cm3 accordingly, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. Several experiments revealed that its stability region is very similar to that of phase A. We further confirmed that this new 23 Å phase was stable in the chlorite composition at 10 GPa and 1000 °C. The present results indicate that this new 23 Å hydrous phase will form in an Al-bearing subducting slab, and transport water together with Al into the deep upper mantle or even into the upper part of the transition zone.