The previously unknown crystal structure of magnesium perchlorate anhydrate, determined and refined from laboratory X-ray powder diffraction data, represents a new structure type. The title compound was obtained by heating magnesium perchlorate hexahydrate at 523 K for 2 h under vacuum and it crystallizes in the monoclinic space group P2(1)/c. The asymmetric unit contains one Mg (site symmetry -1 on special position 2a), one Cl and four O sites (on general positions 4e). The structure consists of a three-dimensional network resulting from the corner-sharing of MgO(6) octahedra and ClO(4) tetrahedra. Each MgO(6) octahedron share corners with six ClO(4) tetrahedra. Each ClO(4) tetrahedron shares corners with three MgO(6) octahedra, with one O-atom corner dangling. The ClO(4) tetrahedra are oriented in such a way that one-dimensional channels parallel to [100] are formed between the dangling O atoms.