The mol-ecular structure of the title compound, C18H21N3S, shows a twisted conformation with a dihedral angle of 67.45 (4)° between the aromatic ring planes and an N-C-C-S torsion angle of -5.01 (13)°. The imidazolidine ring and the aniline moiety make a dihedral angle of 56.03 (4)° and the asscociated C-N-C angle is 125.71 (10)°. The guanidine-like C=N double bond is clearly localized, with a bond length of 1.2879 (14) Å. The C-S-C angle is 102.12 (5)° and the S-C(aromatic) and S-C bond lengths are 1.7643 (11) and 1.8159 (12) Å.