A significant diversity of terrestrial oniscidean isopods was recently discovered in the subterranean ‘calcrete islands’ of Western Australia, but the species and higher-level systematic status of much of the fauna are currently uncertain. Here we focus on one group of species that was initially assigned to the genus Trichorhina (Platyarthridae), based on several shared characters, and investigate the phylogenetic relationships of these species to 21 oniscidean genera, including 13 known families, using 18S rDNA sequence data. We then present phylogenetic analyses using 28S-only and combined 18S, 28S rDNA and mitochondrial cytochrome c oxidase subunit I (COI) data for a more restricted sampling of taxa, and present results for a detailed morphological study of the antennae and other cephalic structures of exemplar taxa. Bayesian and maximum likelihood analyses of the extended 18S-only, the 28S-only and multi-gene datasets provide strong evidence for a distinct well-supported monophyletic group comprising the new Western Australian and one South American taxon. This clade is unrelated to all included members of Platyarthridae, which appears to be polyphyletic, and it forms a distinct group relative to other oniscidean families. Given these findings and the results of the morphological study, a new southern hemisphere oniscidean family, Paraplatyarthridae Javidkar & King, fam. nov. is erected based on Paraplatyarthrus subterraneus Javidkar & King, gen. & sp. nov. (type genus and species), and several undescribed taxa which occur in the arid (terrestrial and subterranean) regions of Western Australia and subtropical South America. Paraplatyarthridae is distinguishable from all other oniscidian families on a combination of character states including, among others, the presence of fan-like scale setae on the dorsal body, and the ventral second antenna with leaf-like scale setae and a furrow containing elongated hair-like capillary setae that form part of a water conducting system unique within Oniscidea. This study has important implications for the higher-level classification of oniscidean crustaceans and points to the need for a more detailed molecular phylogeny that includes a comprehensive sampling of southern hemisphere taxa.