In the title thiosemicarbazone compound, C18H18ClN3S, the CN3S residue is almost planar (r.m.s. deviation = 0.0031 Å) and forms dihedral angles of 65.99 (7) and 34.60 (10)° with the phenyl and chlorobenzene rings, respectively; the dihedral angle between the aromatic rings is 85.13 (8)°. The conformation about the C=N bond is Z, and that about the C=C bonds is E. The imine N and ethyl N atoms are syn and are linked by an ethyl–imine N—H...N hydrogen bond. This H atom also forms an intermolecular hydrogen bond to the thione S atom, resulting in a supramolecular helical chain propagating along the b axis. The chains are consolidated into a three-dimensional architecture by phenyl-C—H...Cl contacts and weak π–π interactions between centrosymmetrically related chlorobenzene rings [inter-centroid distance = 3.9127 (15) Å].