The beet armywom, Spodoptera exigua Hübner, originated from Southeast Asia is one of important agricultural pests worldwide. The moth having no diapause mechanism has highly migratory capacity which contributes to population outbreaks and geographic expansion. Although the moth was first reported in 1926 from Korea, it has been important since the late 1980s. In July 2018, the species outbreaked in Gimje, Jeonbuk Province causing serious damage to soybean in several paddy fields. In this study, we investigate the genetic diversity of the outbreaking populations using a partial sequence of mitochondrial DNA COI gene. 37 COI sequences obtained from specimens collected at the fields of Gimje, Korea were compared with 249 COI sequences from GenBank. In total, 25 haplotypes were detected from 18 countries. Three haplotypes (H1, H2, and H5) found in Korea were all shared with those in China. Haplotypes H1 and H2 were prominent in Korea as well as in invasive regions worldwide. This suggest that outbreaking populations in Korea were invaded from China. The migration source was estimated using backward trajectory analysis, and found to be Zhejiang Province, China.