A new fossil lanternfish of the extant genus Stenobrachius, †S. sangsunii sp. nov., is described from the Middle Miocene deposits of the Duho Formation, South Korea. This is the second fossil species of the genus after the Miocene †S. salnikovi from Karaginsky Island in the Bering Sea. The new species differs from its congeners by the presence of six photophores in ventral series, presence of only one, long supracaudal gland, lesser numbers of the caudal and total vertebrae, greater prepelvic distance, and shorter anal fin base. The presence of a representative of this boreal genus in the Duho Formation fossil assemblage hints at climatic conditions comparable with the modern era during the time of deposition of this formation. This new finding indicates a wide distribution range and substantial morphological diversity of the genus in the North-west Pacific during the Neogene.