Pentacemylus progressus was named by Peterson (1932) [genotype]. Its type specimen is CM 11865, a set of teeth (Upper molars M^ and M^ and lower teeth, including Mi, M2, and part of P4), and it is a 3D body fossil. Its type locality is Leota Ranch, which is in an Uintan terrestrial horizon in the Uinta Formation of Utah. It is the type species of Pentacemylus.