Abstract Lymantria dispar japonica (Lepidoptera: Erebidae), commonly known as the spongy moth, is predominant and widely distributed in Japan. However, between 2021 and 2023, L. d. japonica was simultaneously discovered in several regions of South Korea. Lymantria dispar japonica was distinguished from Lymantria dispar asiatica on the basis of adult forewing length and wing patterns and their genetic distance ranged from 0.065% to 0.588% in the COI gene. Here, we provide a description of this subspecies and details of its sampling locations. The discovery of L. d. japonica is of great importance to the forests of South Korea. Therefore, this study presents an opportunity to investigate new strategies for advancing the efforts to detect and eliminate threats related to this pest in the future.