A new genus and species of bopyrid isopod infests the deep-water galatheid crab Munidopsis beringana collected at a depth of 4100 m off the central coast of California. The head of the female Goleathopseudione bilobatus n. gen., n. sp. is separated from the first pereiomere as in Pleurocryptella, but differs from this and Bathione in having three pairs of lamellae in the barbula and well-developed coxal plates, in the shape of the biramous sub-cylindrical pleopods, and in the absence of uropods. Goleathopseudione is morphologically similar to Pleurocryptella with respect to the absence of lateral plates, and to Bathione with respect to the unsegmented maxillipedal palp. The male of the new genus is close to Pleurocryptella as they both have a well-developed bi-articulated maxillipedal palp, and it is different from it and Bathione in having a sub-ovoid flap covering the sixth segment dorsally. Goleathopseudione is the third genus of bopyrid isopod that infests the host genus Munidopsis in the eastern Pacific, and the second described for a depth greater than 4000 m.