The new mixed-anion phosphate LiMg3(PO4)P2O7 has been synthesized and its crystal structure and ionic conductivity have been studied. The structure of LiMg3(PO4)P2O7 consists of [Mg3O10]∞ layers, with interconnecting PO4 and P2O7 groups that are located between the layers; this represents a new structure type. The Li+ ions are located in the tunnels that extend along the two directions [1 0 0] and [0 1 0]. The lithium ionic conductivity of LiMg3(PO4)P2O7, which is similar to that of LiGd(PO3)4, can be understood by the spacing between the Li sites in the structure. This compound may provide a platform for the design of new Li-containing phosphates for applications in the field of solid electrolytes.