The new acentric indium borate InB6O9(OH)(3) was synthesized in a Walker type multianvil apparatus at extreme pressure and temperature conditions of 12.3 GPa and 1500 degrees C. Single-crystal X-ray diffraction provided the data for the crystal structure solution and refinement. InB6O9(OH)(3) crystallizes in the orthorhombic space group Fdd2 (Z = 8) with the lattice parameters a = 39.011(8), b = 4.4820(9), c = 7.740(2) angstrom, and V = 1353.3(5) angstrom(3). The structure of InB6O9(OH)(3) is basically built of corner-sharing BO4 tetrahedra and isolated InO6 octahedra. The presence of hydroxyl groups was confirmed with vibrational spectroscopic methods (IR and Raman). Furthermore, the second harmonic signal of an InB6O9(OH)(3) powder sample yielded more than twice the intensity of quartz.