Chimaerastacus pacifluvialis, new genus and species, is established on the basis of fossils from the Middle Triassic Liard Formation of northeastern British Columbia. It has well-defined cervical, postcervical, and branchiocardiac grooves; three longitudinal ridges on the cephalic region; and true chelae on the first through third pereiopods. The morphological features of the carapace are a combination of traits used to define both the erymids and the glypheids. A cladistic analysis of 31 decapod genera defines the Astacidea as a monophyletic group, supports the inclusion of the Glypheoidea within the infraorder Astacidea, illustrates the relationships of the Glypheoidea with other astacid groups, and suggests erection of a new family, the Chimaerastacidae, for our new genus and species of decapod. Specimens of C. pacifluvialis are preserved in a sandy bioclastic floatstone that was deposited near the Peace River Embayment in the Middle Triassic. The host lithology suggests that the decapods inhabited a transitional environment between low relief biostromes and the shoreface. This environment provided a unique set of conditions that allowed exceptional preservation of the decapod material.