The genus Yponomeuta is widely known as a forestry insect pest group; however, it is extremely difficult to identify it at the species level because of its small size and morphological complexity. Thus far, only a few major species and host plants, as well as natural enemy information, are known through European research within the limited eastern Palearctic region. In this study, Yponomeuta meguronis Matsumura is revealed to be a severe forestry insect pest for Euonymus japonicus Thunb. (Celastraceae) in Korea, which constitutes part of the western Palearctic region. We observed that numerous tiny larvae caused serious damage to its leaves and partially or entirely covered the tree by making communal silk webs. To rapidly and accurately identify this insect pest in the future, new descriptions of immature stages, including larva and pupa, were provided, along with their adult forms, based on ecological photographs and morphological illustrations. Two natural enemies of Y. meguronis were also recognized for the first time in this study: Herpestomus brunnicornis (Gravenhorst) (Hymenoptera: Ichneumonidae) was revealed as a parasitoid, and Xanthandrus comtus (Harris) (Diptera: Syrphidae) was presented as a predator.