In the crystal structure of the title compound, C5H6N3O2 +·H2PO4 −, the dihydrogen phosphate anions are linked through short O—H⋯O hydrogen bonds, forming infinite double chains running parallel to the b axis. Centrosymetric N—H⋯O hydrogen-bonded cationic dimers form bridges between these chains by means of intermolecular N—H⋯O and O—H⋯O hydrogen bonds, leading to a two-dimensional network parallel to (100) in which R 3 3(12), R 4 3(10) R 2 2(8) and C(4) graph-set motifs are generated. Weak intermolecular C—H⋯O hydrogen bonds connect these layers, forming a three-dimensional network.