The red-necked longhorn beetle (RLB; Aromia bungii [Faldermann, 1835]) is an emerging invasive pest. From its native range of East Asia, it invaded Europe and Japan in the early 2010s. Despite its increasing importance, the molecular resources of RLB are scarce, and its invasive dynamics are largely unknown. In the present study, we carried out the first analysis of its population genetic structure in South Korea, which is part of its native range, using 1248 bp cytochrome oxidase subunit I (COI) sequences of 199 individuals from 18 localities. We found that in South Korea, RLB has a moderate population genetic structure and can be divided into three geographical subgroups: central, southeastern, and southwestern subgroup. Comparative analyses with two Chinese, one German, and ten Italian RLB sequences yielded non-significant results because of largely missing genetic data from other native areas. Nevertheless, as it provided the first population genetic data for this invasive alien species (IAS) whose range is increasing, our research is a crucial molecular resource for future invasive dynamics research.