The new high-pressure gallium borate Ga₂B₃O₇(OH) was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 10.5 GPa and 700 °C. For the system Ga–B–O–H, it is only the second known compound next to Ga₉B₁₈O₃₃(OH)₁₅·H₃B₃O₆·H₃BO₃. The crystal structure of Ga₂B₃O₇(OH) was determined by single-crystal X-ray diffraction data collected at room temperature. Ga₂B₃O₇(OH) crystallizes in the orthorhombic space group Cmce (Z = 8) with the lattice parameters a = 1050.7(2) pm, b = 743.6(2) pm, c = 1077.3(2) pm, and V = 0.8417(3) nm³. Vibrational spectroscopic methods (Raman and IR) were performed to confirm the presence of the hydroxyl group. Furthermore, the band gap of Ga₂B₃O₇(OH) was estimated via quantum-mechanical density functional theory calculations. These results led to the assumption that our gallium borate could be a suitable substance to split water photocatalytically, which was tested experimentally.