In the title compound, [Ag(C(10)H(9)N)(H(2)O)](C(6)H(6)NO(3)S), the Ag(I) atom is two-coordinated by one N atom from a 3-methyl-isoquinoline ligand and one water mol-ecule. The 4-amino-benzene-sulfonate counter-anion does not show any bonding inter-actions with the Ag(I) atom. The compound exhibits a three-dimensional supra-molecular structure constructed by hydrogen bonds. Adjacent isoquinoline groups form π-π inter-actions, with a centroid-centroid distance of 3.54 (1) Å. The crystal studied was an inversion twin.