The absolute structure of the title compound, [H(3)N-(CH(2))(7)-NH(2)]I, has been determined from the diffraction experiment, the Flack parameter refining to -0.02 (2). In the crystal, adjacent symmetry-related cations are connected by head-to-tail R'H(2)N(+)-H⋯NH(2)R hydrogen bonds, forming chains along [010]. The remaining four H atoms attached to the amino and the aza-nium group form weak hydrogen bonds to neighbouring iodide anions, producing a three-dimensional hydrogen-bonded network. The amino group and the aliphatic chain of the 7-amino-heptyl-aza-nium cation show an exact all-trans conformation, within experimental uncertainties. The aza-nium group, to fulfill the needs of hydrogen bonding, is twisted out of the plane defined by the C atoms of the aliphatic chain, the C-C-C-N torsion angle being -65.4 (4)°.