Brentidae, a family of primitive weevils, is associated with dead woods mainly in the tropical rain forests and includes approximately 1,750 species worldwide. The genus Cyphagogus has been known more than 60 species from Madagascar to Samoa and Fiji islands and from Japan to Australia and Tasmania. Twelve species were previously recognized from Asia and Oriental region, among them three species from Japan, two from China. One male of the genus Cyphagogus was collected during faunal studies of wood-boring insects in the Gwangneung forest of South Korea in 2011. The species was identified as C. iwatensis Morimoto, which was described on two female specimens in 1976. This is the first discovery of the male of C. iwatensis as well as the first record of the genus and species in the country. Diagnosis and photographs of the detailed diagnostic characters including line drawing of male genitalia are provided in the present study, with a key to the East Asian species of bipunctatus-group.