Diamondback moth, Plutella xylostella L. (Plutellidae: Lepidoptera) is one of the most important insect pests, constraining brassica production worldwide. Since it has developed resistance to chemical and bio-pesticides, no single approach can provide satisfactory control of this notorious pest. Hence, integrated pest management strategies are warranted. In order to use the control options such as bio-control agents and sex pheromone lures, a thorough understanding of the population structure of the target pest is highly imperative. Hence, this study was undertaken to assess the genetic diversity of the P. xylostella population in Southeast Asia, especially in Cambodia, Lao PDR, Malaysia, Thailand, Taiwan and Vietnam. For comparison, a population from West Asia (Syria) was also included. The cytochrome c oxidase subunit 1 (cox1) gene was used to understand the phylogenetic relationship of geographically different P. xylostella population from Southeast Asia. Extensive sampling was done from different host plant species (broccoli, cabbage, cauliflower, Chinese cabbage, Chinese kale, green mustard, pak-choi, radish, and kohlrabi) in target countries. A total of 52 different populations containing 245 individuals were used in the study. An amplicon of 709 bp was produced by polymerase chain reaction, and editing resulted in a consensus sequence of 643 bp across all P. xylostella population. A total of 77 haplotypes were identified in 245 P. xylostella individuals. Phylogenetic analysis showed no difference among most of the P. xylostella population from different host plants, except few populations from Thailand and Vietnam, which formed a separate cluster. The high FST values (0.44- 0.45) of P. xylostella population in Taiwan compared to Cambodia and Malaysia population seem to indicate the presence of genetically diverse populations, which are yet to be confirmed by additional sampling. The negative Tajima’s D and Fu’s FS values showed the recent demographic expansion of the P. xylostella population in Cambodia, Lao PDR and Vietnam. Thus, this study showed the likely presence of genetically diverse P. xylostella in Southeast Asia, but it requires additional studies with more populations, especially from Cambodia, Lao PDR, Vietnam, Thailand and Taiwan.