The title compound, (CH6N3)7H9[PtMo6O24]2·7H2O, containing the well-known Anderson-type heteropolyoxomolybdate, was obtained by recrystallization of its powdered guanidinium salt. The protonated O atoms in the polyanion were confirmed by electron-density maps, interpolyanion hydrogen bonds and bond-valance sums (BVS). The {[H4.5PtMo6O24]2}7− polyanion is the same as that already characterized in K7[H4.5PtMo6O24]2·11H2O [space group P-1; Lee & Joo (2010). Acta Cryst. E66, i8–i9]. The heteropolyanions form inversion-generated dimers, {[H4.5PtMo6O24]2}7−, held together by each of the four μ3-O—H...μ1-O, two μ2-O—H...μ2-O hydrogen bonds and one centrosymmetric μ3-O—H—μ3-O hydrogen bond. The H atom of the centrosymmetric hydrogen bond is located on an inversion centre. One guanidinium ion and one water molecule are equally disordered about a twofold rotation axis.