Naphtho[2,3-b]thiophene diimide (NTI) was found to be selectively thionated to afford naphtho[2,3-b]thiophene-4,9-dicarboxy-5,8-dithiocarboxydiimide (NTI-2S). By elucidating its electrochemical, optical, and structural properties, the effects of thionation on the molecular electronic structure and crystal structure were discussed. Although thionation can greatly lower the LUMO energy level and enhance the intermolecular interaction in the solid state, its chemical lability is a drawback as an organic semiconducting material.