Gnathia limicola sp. nov. is described from Okinawajima Island, Ryukyu Archipelago, southwestern Japan. Burrows of this species were found in a small intertidal creek bank on a muddy tidal flat near mangrove trees. Adult males differ from those of other Gnathia species in the following features: (1) fine setae cover peduncle articles 1 and 2 of antenna 1, peduncle articles 1–3 of antenna 2, and the erisma of the mandibles; (2) the ventral frontal border of the cephalon is medianly notched, and the lateral parts extend beyond the dorsal frontal border; and (3) the penes are fused into a thin rectangular blade directed posteriorly. Adult females and praniza larvae were also distinguished morphologically from other Gnathia species. Based on field and laboratory observations, the mating behavior of this species appears similar to that of Paragnathia formica (Hesse, 1862), which inhabits salt marshes in Europe and North Africa.