New semi-aromatic polythioamides have been synthesized, based on aliphatic diamines and a new bis(dithioester) derived from 2,6-naphthalenedicarboxylic acid. Polythioamides were prepared by solution polycondensation at low temperature, in high yields and moderate to high inherent viscosities. The presence of naphthalene rings instead of benzene rings or aliphatic groups affected the thermal transitions in these polythioamides. By comparison with aliphatic or benzene polythioamides, polythioamides containing naphthalene revealed lower glass transition temperatures and a tendency to form liquid crystal mesophases. Noticeable solubility improvements were also observed.