In the crystal structure of the title compound, C(5)H(6)N(3)O(2) (+)·H(2)PO(4) (-), 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 inter-molecular 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 inter-molecular C-H⋯O hydrogen bonds connect these layers, forming a three-dimensional network.