The title compound, (C₅H₉N₂)₄[Mo₈O₂₆]·4H₂O, was reported previously from a room-temperature data collection from which only the metal atoms could be refined anisotropically [FitzRoyet al.(1989).Inorg. Chim. Acta,157, 187–194]. The current redetermination at 180 (2) K models all the non-H atoms with anisotropic displacement parameters and fully describes the supramolecular N—H...O and O—H...O hydrogen-bonded network connecting the 3,5-dimethylpyrazolium cations, the water molecules of crystallization and the β-octamolybdate anion. All H atoms involved in the three-dimensional hydrogen-bonding network could be located from difference Fourier maps, with the exception of those of one disordered water molecule, firstly seen in this structural report [refined over two distinct locations with site-occupancy factors of 0.65 (2) and 0.35 (2)]. The complete β-octamolybdate anion is generated by a crystallographic inversion centre.