The crystal structure of A-site deficientLa4Mg3W3O18 perovskite has been solved by x-ray powder diffraction in combination with grouptheoretical analysis. Above 700 K, the crystal structure is orthorhombic (space groupIbam;2ap × 4ap × 2ap type superstructure) and presents a sequence of[LaO]–[Mg1/2W1/2O2]–[La1/3O]′–[Mg1/2W1/2O2]–[LaO]–[Mg1/2W1/2O2]–[La1/3O]′′–[Mg1/2W1/2O2] layers stackedalong the b axis. The lanthanum ions and the vacancies in the[La1/3O]′ and [La1/3O]′′ layers are ordered and form rows along thec axis. A half-periodshift along the a direction between these layers leads to a quadrupling of the primitive perovskite unit cell in theb direction.The ordering of the vacancies in the lanthanum poor layers is connected with the ionic ordering betweenMg2+ andW6+ in theneighbouring [Mg1/2W1/2O2] blocks. Around 700 K, due to an anti-phase rotation of the octahedra, a continuous phase transition mediatedby the Γ2+ irreducible representation from orthorhombic(Ibam,a0b0c0) tomonoclinic (C 2/m, a0b0c−) symmetry takes place.