In the structure of the title salt C12H11N2S+·I−, the methylsulfanyl group of the cation is nearly coplanar with the perimidine rings, as indicated by the C—S—C—N torsion angles of 2.9 (5) and −177.2 (3)°, respectively. The (S)C—N bond lengths in the heterocyclic ring are approximately equal [1.325 (5) and 1.326 (6) Å] suggesting a degree of delocalization. In the crystal, cations and anions are linked via two discrete N—H...I hydrogen bonds, forming chains along the b axis.