Lack of an ischium on pereiopods 1–6 was previously considered a synapomorphy for the tanaidacean superfamily Tanaoidea, although descriptions of Arctotanais alascensis (Richardson, 1899), the sole species in Arctotanais, indicated presence or absence of the ischium. To resolve this ambiguity, we examined newly collected specimens of A. alascensis (including males, which had not previously been described) from Hokkaido, Japan, using light and scanning electron microscopy. We also conducted molecular phylogenetic analyses based on partial sequences of the 18S rRNA gene to examine the phylogenetic position of A. alascensis. Here we describe in detail the morphology of the male of A. alascensis, which proved to be similar to that of the females. This species bears an ischium on pereiopods 1–6, which contradicts the current diagnoses of Tanaoidea and Tanaidae, although other synapomorphies remain valid. Molecular phylogenetic analyses strongly supported the placement of A. alascensis in Tanaoidea, and consequently we amended the diagnoses for Tanaoidea and Tanaidae to include either presence or absence of the ischium on the pereiopods.