Mysidobdella californiensis n. sp. is described from the mysid Holmesimysis sculpta from Bodega Bay on the central California coast and from Holmesimysis costata var. from San Pedro on the southern California coast. The internal anatomy of M. californiensis is similar to that of the only other species in the genus, Mysidobdella borealis from the north Atlantic Ocean, except that M. californiensis lacks the medial, unpaired seminal receptacle present in M. borealis. Externally, M. californiensis is slightly larger and more robust than M. borealis, with a much larger caudal sucker. The most striking difference between the species is the unusually large, trumpet-shaped, fluted oral sucker in M. californiensis. At Bodega Bay, the prevalence of M. californiensis on its host was 17% with an average intensity of 1.46 (range 1–3) leeches per host.