A new species of halophilous water strider, Aquarius haliplous Yasunaga, Asanabe, and Nagashima new species (Hemiptera: Gerridae: Gerrinae), is diagnosed and described. This species is endemic to Omura Bay, Nagasaki, southwestern Japan, and has a unique adaptation to this mixohaline habitat. Based on unique morphology and ecology both in the adult and immature forms, this new Aquarius Schellenberg species can be separated unequivocally from a similar species inhabiting freshwater, A. paludum (Fabricius). A key is provided to facilitate identification of the Japanese species and subspecies of Aquarius.