A new neritiliid gastropod, Neritilialittoralis, lives in a unique, intertidal, subterranean, brackish environment on Amami-ohshima Island, Japan. It lives exclusively on rubble that is embedded deeply in coarse-grained sand whose interstices are filled with a mixture of subsoil water and seawater. Empty shells of this species were also found in tide pools at the foot of limestone terraces on Okinawa Island, where a large volume of karst-related freshwater gushes out. The species is characterized by a very small, white shell and unpigmented skin. Neritilia has previously been found in freshwater streams, rivers, estuaries, and in anchialine and phreatic waters, and the present find expands its range of known habitats and has implications for the history of the invasion of freshwater by neritiliids. The spiral protoconch suggests that the larva of N.littoralis is long-lived, planktotroph and a very effective disperser, and all the individuals collected appear to be part of a panmictic population.