A charge-assisted hydrogen-bonding network involving N-H⋯O and O-H⋯O hydrogen bonds stabilizes the crystal of the title salt, C5H8N3 (+)·H2PO4 (-). The dihydrogen phosphate anions form one-dimensional chains along [100], via O-H⋯O hydrogen bonds. The 2-amino-4-methyl-pyrimidinium cations are linked to these chains by means of two different kinds of N-H⋯O hydrogen bonds. Neighbouring chains are linked via C-H⋯N and C-H⋯O hydrogen bonds forming two-dimensional slab-like networks lying parallel to (01-1).