Protamphisopus wianamattensis (Chilton, 1918) from the Middle Triassic (Anisian) Ashfield Shale in the Sydney Basin, Australia, is the earliest-known freshwater representative of the basal isopod suborder Phreatoicidea. In contrast, the late Paleozoic Palaeophreatoicidae species, which are morphologically distinct from extant families, are found in marine or estuarine facies. Comparison of Protamphisopus wianamattensis with living Phreatoicidea permits the external morphology of the fossils to be reconstructed and the species to be coded for cladistic analysis using a revised and expanded character set developed for living phreatoicideans. In resulting parsimonious trees as well as immediately suboptimal trees, Protamphisopus is nested within clades related to the family Amphisopodidae. Although not included in the analysis, the Late Permian Protamphisopus reichelti Malzahn (in Glaessner and Malzahn, 1962) appears to be a member of the Palaeophreatoicidae, rather than among the crown group of the Phreatoicidea. Therefore, a minimum age of Middle Triassic can be assigned to the basal branches within the phreatoicid crown group. The minimum age for the colonisation of fresh water by the suborder is also established although, given the advanced position of Protamphisopus wianamattensis in the cladograms, the habitat shift may have occurred earlier. The biogeographic distribution of extant Phreatoicidea on fragments of Gondwana is consistent with early Mesozoic origins for the major clades of this isopod suborder.