The almost planar (r.m.s. deviation = 0.055 Å) title compound, (MeS)C(O)NHC(O)NH2, was formed during an attempted crystallization of dimethyl cyanocarbonimidodithioate with CrO2Cl2; an unexpected redox reaction converted the cyanocarbonimido moiety to a urea group and removed one methylthiol group. In the crystal, hydrogen-bonding interactions from the amide and amido N—H groups to carbonyl O atoms of neighbouring molecules result in [010] ribbon-like chains.