The cation of the title compound, C(7)H(7)N(2)S(+)·H(2)PO(4) (-), is almost planar (r.m.s deviation = 0.017 Å for all non-H atoms). In the crystal structure, the cations and anions are connected by N-H⋯O and O-H⋯O hydrogen bonds, with π-π stacking inter-actions between neighbouring 1,3-thia-zole and benzene rings [centroid-centroid distance = 3.5711 (11) Å], forming a three-dimensional network.