The genus Deltamysis, in the tribe Mysidetini (Mysidae: Heteromysinae), previously contained a single species, D. holmquistae, before two additional species, D. nana and D. songkhlaensis, were transferred from Heteromysoides of the related tribe Heteromysini. A new member of the genus, D. lowryi sp. nov., found at the mouth of the Clarence River, New South Wales, in 1988, is described here from the Australian Museum collection. The collection also contained two specimens of D. holmquistae from the eastern Australian coasts of the Coral Sea and the Tasman Sea, not previously recorded from Australia. Deltamysis lowryi sp. nov. differs from the other three species of the genus by the structure of the telson, which has a wide, shallow cleft, armed with a number of spinules, and notably shortened subterminal spiniform setae. The diagnosis and the generic composition of the tribe Mysidetini, and the diagnoses of the genus Deltamysis and all its four species are updated. Deltamysis has so far been the only genus of the marine subfamily Heteromysinae diversifying in brackish estuarine water. The genus has a clear western Indo-Pacific natural occurrence, but has been introduced also to the Eastern Pacific and more recently to the Atlantic coasts of North America.