A new species, Monoblastus forsythia sp. nov. is described from South Korea. All specimens of this new species were collected on Forsythia koreana (Nakai). A taxonomic study of South Korean Monoblastus was initiated by Uchida in 1930, and until now four species have been recorded from South Korea. The genus Monoblastus is a Holarctic genus with 31 described species, 14 species are Nearctic, seven are Western Palaearctic region, and nine species occur in Eastern Palaearctic region. In the study, I provide description and photos of both sexes of this new species. Also, modified key to species of the South Korean Monoblastus is provided.