The land snail Ellobium chinense (Pulmonata, Ellobiida, Ellobiidae), which inhabits the salt marshes along the coastal areas of northwestern Pacific, is an endangered species on the IUCN Red List. Over recent decades, the population size of E. chinense has consistently decreased due to environmental interference caused by natural disasters and human activities. Here, we provide the first assessment of the genetic diversity and population genetic structures of northwestern Pacific E. chinense based on COI and 10 microsatellite markers. The analyses of 140 COI data from South Korea and Japan and 54 microsatellite data from South Korea revealed that E. chinense has high haplotype and low nucleotide diversity without showing any genetic structures that reflect geographical isolations. It strongly implies that the subfamily Ellobiinae may have first appeared around the Eocene Optimum immediately after the Paleocene–Eocene Thermal Maximum (PETM; ca. 55 mya) and the examined E. chinense populations in Northwestern Pacific may have been maintained in a metapopulation under the influence of the Kuroshio warm currents through the Late-Middle Pleistocene (0.350−0.126 mya) and Late Pleistocene (0.126−0.012 mya). We also found four phylogenetic groups, regardless of geographical distributions, which were easily distinguishable by four unidirectional and stepwise adenine-to-guanine transitions in COI (sites 207-282-354-420: A-A-A-A, A-A-G-A, G-A-G-A, and G-G-G-G). Additionally, the four COI hotspots were robustly connected with a high degree of covariance between them. We discuss the role of these covariate guanines which link to form four consecutive G-quadruplexes, and their possible beneficial effects under positive selection pressure.