Aluminum hydroxysulfate, AlSO4(OH), is postulated to play a vital role in controlling the solubility of aluminum in sulfate-rich acidic soils and ground waters, but it has not yet been confirmed in nature. This study reports the synthesis of an AlSO4(OH) crystal at 700 °C and ~1.0 GPa in a hydrothermal diamond-anvil cell from a mixture of 95% H2SO4 and Al2O3 powder and its structure determination from single-crystal X-ray diffraction data. AlSO4(OH) is monoclinic with space group C2/c and unit-cell parameters a = 7.1110(4), b = 7.0311(5), c = 7.0088(4) Å, β = 119.281(2)°, and V = 305.65(3) Å3. Its crystal structure is characterized by kinked chains of corner-sharing AlO6 octahedra that run parallel to the c-axis. These chains are linked together by SO4 tetrahedra and hydrogen bonds, forming an octahedral-tetrahedral framework. Except for the numbers and positions of H atoms, AlSO4(OH) is isostructural with the kieserite-type minerals, a subgroup of the titanite group of minerals. A comparison of powder X-ray diffraction patterns indicates that our AlSO4(OH) is the same as that obtained by Shanks et al. (1981) through hydrolysis of Al2(SO4)3 solutions at temperatures above 310 °C. To date, AlSO4(OH) has been synthesized only at temperatures above 290 °C, implying that it may not stable in low-temperature environments, such as acidic soils and mine waters. The possible environments to find Al(OH)SO4 may include places where sulfur-rich magma-derived fluids react with aluminous rocks under elevated temperature and pressure, and on Venus where a sulfur-rich atmosphere interacts with surface rocks at temperatures above 400 °C.