A new genus and a new species of ant, Proleptothorax primitivus, are described based on two males from the Late Eocene Rovno amber of Ukraine. This genus is characterized by the antennae 13-segmented, with the funiculus filiform without an apical club, by the very short antennal scape, by the short and narrow, bidentate mandibles, and by the forewing with cell 3r closed. We consider these features as obvious plesiomorphies compared with Leptothorax Mayr, 1855, Formicoxenus Mayr, 1855, and Harpagoxenus Forel, 1893 and assume that Proleptothorax could be regarded as a putative ancestor of the extant genera of the Leptothorax genus group.