The title compound, (C(4)H(12)N(2))(3)[PMo(12)O(40)] or (H(2)pip)(3)[PMo(12)O(40)] (pip is piperazine), was prepared under hydro-thermal conditions. The asymmetric unit contains one-sixth of a mixed-valent Mo(V,VI) pseudo-Keggin-type [PMo(12)O(40)](6-) anion and half a piperazinediium cation, (H(2)pip)(2+). The discrete Keggin-type [PMo(12)O(40)](6- )anion has site symmetry and the three (H(2)pip)(2+) cations each have site symmetry at the centres of the mol-ecules. The central P atom is on special position , which is a roto-inversion position and generates the disorder of the PO(4) tetra-hedron. Furthermore, six doubly bridging oxide groups are also disordered with an occupancy factor of 0.5 for each O atom. The anions and cations are linked by an extensive network of inter-molecular N-H⋯O and C-H⋯O hydrogen bonds.