The asymmetric unit of the title molecular salt, [C5H7N2+][(HO)2OP(CH2)9PO2(OH)−], consists of one 4-aminopyridinium cation and one hydrogen (9-phosphonononyl)phosphonate anion, both in general positions. As expected, the 4-aminopyridinium moieties are protonated exclusively at their endocyclic nitrogen atom due to a mesomeric stabilization by the imine form which would not be given in the corresponding double-protonated dicationic species. In the crystal, the phosphonyl (–PO3H2) and hydrogen phosphonate (–PO3H) groups of the anions form two-dimensional O—H...O hydrogen-bonded networks in the ab plane built from 24-membered hydrogen-bonded ring motifs with the graph-set descriptor R66(24). These networks are pairwise linked by the anions' alkylene chains. The 4-aminopyridinium cations are stacked in parallel displaced face-to-face arrangements and connect neighboring anionic substructures via medium–strong charge-supported N—H...O hydrogen bonds along the c axis. The resulting three-dimensional hydrogen-bonded network shows clearly separated hydrophilic and hydrophobic structural domains.