The title compound, C13H10Cl2N2S, represents a monoclinic polymorph of the previously reported orthorhombic form [Ramnathan et al. (1996 ). Acta Cryst. C52, 134–136]. The molecule is twisted with the dihedral angle between the benzene rings being 55.37 (7)°. The N—H atoms are syn to each other, which contrasts their anti disposition in the orthorhombic form. In the crystal, molecules assemble into zigzag chains along the c axis via N—HS hydrogen bonds. Chains are connected into layers via C—HCl interactions, and these stack along the a axis.