Polyphrix stellata sp. nov., from the Brazilian Amazon Forest, is described and illustrated. The validity of the genus is cladistically tested using implied weighting, in a matrix with 26 species and 75 morphological characters. Concavity constants (K) from 1 to 6 were tested. All analyses recovered all known Polyphrix species as a monophyletic group. The preferred results, with K2–6, suggest (Prosthoporus nigrifemur Gupta + Lagarosoma assitum Gupta) + Messatoporus sp. as the sister group of the genus; with K1, this group also included two species of Gabuniina. The results with K1–5 suggest an Amazonian origin of the genus, with species from northern Brazil appearing basally in the clade, while both Atlantic Forest species appear as sister groups. A redescription of Polyphrix is presented, combining the original definition with recently published additions and original information. An updated key to species is presented, and new morphometric data is provided for P. varians and P. cristata. Prosthoporus mexicanus comb. nov. is proposed as a new combination for Polyphrix mexicanus Kasparyan & Ruíz, representing the first record of Prosthoporus from North America. Seven new distribution records are provided for Polyphrix, expanding its known range to 29º51' of latitude, from Guyana to Paraná (southern Brazil).