In the asymmetric unit of the title compound, C10H15N4O2+.H2PO4-, there are two protonated aminoguanidinium cations and two dihydrogenphosphate anions. The positive charge on the protonated amidine group is delocalized over the three C-N bonds in a manner similar to that found in guanidinium salts. The aminoguanidinium cations are found to be the E-isomer structures. Intramolecular interactions of the N-H...N type are observed, leading to the formation of five-membered rings. Extensive networks of O-H...O, N-H...O and C-H...O hydrogen bonds stabilize the three-dimensional network. In the crystal structure, pi-pi interactions between the benzene rings, with a distance of 3.778 (2) angstroms between the ring centroids, also affect the packing of the molecules.