Synthetic gengenbachite, KH8(Fe2.66Al0.34)(PO4)6·6H2O, has been prepared hydrothermally at 423–393 K. Single-crystal X-ray structure refinements (R1 = 0.035 for 1838 [I > 2σ(I)] unique reflections and wR2 = 0.083 for all 1863 reflections) confirm that gengenbachite crystallizes in the trigonal space group P31c (No. 159) and is isotypic with synthetic (NH4) H8Fe3+3(PO4)6·6H2O. The crystal structure consists of [FeO6] octahedra and [PO4] tetrahedra forming a three-dimensional framework with eight-membered-ring channels running along the , , and  directions. All but one hydrogen atom have been located to allow description of the hydrogen bonds in gengenbachite. Magnetic susceptibility analysis of gengenbachite yielded the Curie constant C = 12.07 emu·K·mol−1 and the Weiss constant θP = −58.9 K. The effective magnetic moment per Fe atom μeff = 6.03 μB provides further support for high-spin Fe3+ in octahedral coordination.