The crystal structure of the title compound containing lutetium, the last element in the lanthanide series, was determined using a single crystal prepared by heating a pressed pellet of a 2:1 molar ratio mixture of Lu2O3 and Al2O3 powders in an Ar atmosphere at 2173 K for 4 h. Lu4Al2O9 is isostructural with Eu4Al2O9 and composed of Al2O7 ditetrahedra and Lu-centered six- and sevenfold oxygen polyhedra. The unit-cell volume, 787.3 (3) Å3, is the smallest among the volumes of the rare-earth (RE) aluminates, RE4Al2O9. The crystal studied was refined as a two-component pseudo-merohedric twin.