Platycorynus peregrinus (Herbst, 1783) is a leaf beetle of agricultural importance, yet its genetic diversity and population structure remain poorly understood. We analyzed mitochondrial cytochrome c oxidase subunit I (COI) sequences from 147 individuals across 19 populations in Thailand. Forty-five haplotypes were identified, showing high haplotype diversity (0.942) and moderate nucleotide diversity (0.00562). Significant genetic differentiation (Φ_ST = 0.0000–0.7857) was strongly associated with geographical distance, and this population structure was further supported by AMOVA (F_CT = 0.21925, p < 0.001). Neutrality tests and mismatch distribution analyses revealed signals of population expansion, alongside evidence of localized differentiation. The substantial genetic variability observed suggests high dispersal ability and possible cryptic lineages. These insights not only advance understanding of the evolutionary dynamics of P. peregrinus but also provide valuable guidance for developing sustainable pest management strategies.