In the title compound, [Ag(C10H9N)(H2O)](C6H6NO3S), the AgI atom is two-coordinated by one N atom from a 3-methylisoquinoline ligand and one water molecule. The 4-aminobenzenesulfonate counter-anion does not show any bonding interactions with the AgI atom. The compound exhibits a three-dimensional supramolecular structure constructed by hydrogen bonds. Adjacent isoquinoline groups form π–π interactions, with a centroid–centroid distance of 3.54 (1) Å. The crystal studied was an inversion twin.