The Russian Far East is known for its complicated geological, paleogeographical history, and diverse biota. Detailed biogeographical data of freshwater mollusks, for both insular and adjacent mainland regions, is made available. Kuril Archipelago and Sakhalin Island represent the largest of the insular territories. They are characterized by diverse, but not relict, malacofauna. Hokkaido is the southern source for that insular malacofauna. From the north, Sakhalin Island is colonized by a freshwater malacofauna of the Low Amur River and the Tatar Strait continental drainages. The northern source of the Kuril malacofauna is Kamchatka. Ten distinguished regions are designated using UPGMA cluster analysis that looked for similarities among 228 freshwater mollusk species. Bootstrap analysis is used to insure the accuracy of each cluster, which resulted in two major assemblages with similarity indexes close to zero: Palaearctic and Sino-Indian malacogeographical regions separated by Kuril’s Bussol Strait. On the Sakhalin Island, the Sino-Indian region is devided by a secondary boundary called Schmidt's line. The northern part of the island is inhabited by an Amurian freshwater malacofauna, and the southern part is mainly inhabited by Japanese malacofauna. This malacogeographical subdivision correlates well with the distribution patterns of other organisms including plants and mushrooms.