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Single crystals of the title compound, K2Al2P8O24, were obtained by solid-state reaction. The monoclinic structure is isotypic with that of the GaIII analogue and is built of eight-membered phosphate ring anions P8O248− (2/m symmetry) isolated from each other and further linked by isolated AlO6 octahedra (overline{1} symmetry) by sharing corners. Each AlO6 octahedron is linked to four P8O248− rings in such a way that two rings are linked through bidentate diphosphate groups attached in the cis positions on two opposite parallel edges of the octahedron. The two other rings are linked via corner-sharing to the two remaining corners in the trans positions of the AlO6 octahedron. Each P8O248− ring anion is linked to eight AlO6 octahedra. More accurately, each ring anion is linked to four AlO6 octahedra through bidentate diphosphate groups attached in the cis positions to the AlO6 octahedron and to the four remaining octahedra by sharing corners. This three-dimensional linkage delimits channels running parallel to [001] in which the ten-coordinated K+ cations (2 symmetry) are distributed over two columns. These columns alternate with empty octagonally-shaped channels expanding through the P8O248− ring anions.