Nemobius tertiarius was named by Scudder (1878). Its type specimen is 18, a set of appendages (hind femur & hind tibia), and it is a compression fossil. Its type locality is Petrified fish cut (Scudder collection), which is in a Bridgerian lacustrine - large diatomite/shale in the Green River Formation of Wyoming.

It was recombined as Pronemobius tertiarius by Scudder (1890), Handlirsch (1907), Zeuner (1939), Gorochov (2012), Heads et al. (2023).