Based on mitochondrial DNA barcoding, we identified the Dianbai oyster Crassostrea dianbaiensis from the Hiki River in Wakayama Prefecture, which is located on the Kii Peninsula on the Pacific coast of central Japan. Our sampling site represents the northern- and easternmost habitat of this species reported to date. Out of our 21 Crassostrea specimens examined, 20 were identified as C. dianbaiensis with one individual of the Pacific oyster C. gigas. In the field, C. dianbaiensis attached intertidally and subtidally to concrete blocks in brackish water (approximately 1 km upstream from the river mouth), where Saccostrea spp., C. gigas, Isognomon ephippium and Hormomya mutabilis were found in sympatry. Our specimens showed various shell sizes, implying that C. dianbaiensis successfully reproduces at the newly found habitat. Although C. dianbaiensis has so far been considered a tropical or subtropical oyster, there is a possibility that this species recently extended its habitat northward, in line with the increasing observations of southern species of marine benthos along the southwestern coast of the Kii Peninsula since the 1980s.