This research introduces the subfamily Catotrichinae to the South Korean fauna for the first time. Within the globally recognized 6,651 Cecidomyiidae species, only ten are categorized under the Catotrichinae subfamily. Notably, this subfamily, which ingests fungi during larval development, is among the most primordial lineages of the Cecidomyiidae, both in morphological and molecular terms. The species Catoricha nipponensis of Catotrichinae was newly observed in Yeongwol, Gangwon-do, in October 2021. It was recorded for the first time in Korea, with its holotype initially collected in Honshu, Japan, in November 1923. This study provides the diagnosis, photographs of distinguishing characteristics, and the DNA barcode sequences for Catotricha nipponensis. This work was supported by a grant from the National Institute of Biological Resources (NIBR), funded by the Ministry of Environment (MOE) of the Republic of Korea.