A new aluminum borate, Ba[AlB4O8(OH)], has been synthesized under high-temperature, high-pressure hydrothermal conditions at 550 °C and 1400 bar and its structure characterized by single-crystal X-ray diffraction, IR, and MAS 11B, and 27Al NMR spectroscopy. It crystallizes in the monoclinic space group P21/n with a = 7.0695(5) Å, b = 15.108(1) Å, c = 7.0746(5) Å, β = 93.593(2)°, and Z = 4. Its 2D layer structure is formed of corner-sharing B4O8(OH) clusters and AlO4 tetrahedra with the charge-compensating Ba2+ cations between the layers. While the same in the framework composition, the title compound and the hydrate, Ba[AlB4O8(OH)]·H2O, differ greatly in structure. Although the title compound contains an OH group, it is thermally stable up to 740 °C and then decomposes into Ba2Al2B8O17, as indicated by high-temperature DSC/TG analysis and powder X-ray diffraction.