The asymmetric unit of the title compound, bis[4,4′-methylenebis(3,5-dimethyl-1H-pyrazol-2-ium)] β-octamolybdate, (C11H18N4)2[Mo8O26] or (H4mbdpz)2[Mo8O26], is composed of an H4mbdpz2+ cation and half of the β-octamolybdate anion which is completed by inversion symmetry. The organic molecular units are engaged in a series of N—H...O hydrogen bonds with neighbouring anions, with N...O distances and N—H...O angles in the ranges 2.730 (2)–2.941 (2) Å and 122–166°, respectively. These interactions lead to the formation of a supramolecular two-dimensional network parallel to the (010) plane.