<p><i>Gymnema sylvestre</i> is an important industrial medicinal plant in India, is valued for its anti-diabetic properties. However, its cultivation is severely affected by the oleander aphid, <i>Aphis nerii</i> Boyer de Fonscolombe (Hemiptera: Aphididae), a destructive pest causing significant yield losses. Despite its economic impact, the population genetics of <i>A. nerii</i> remain poorly understood. This study examined the genetic structure, diversity, and demographic patterns of <i>A. nerii</i> using 78 sequences of the mitochondrial cytochrome oxidase subunit I (COI) gene. Eight haplotypes were identified, with four (Hap_1 to Hap_4) prevailing in India. Notably, Hap_3 was widely distributed across Japan, Costa Rica, Australia, Canada, Pakistan, and China, suggesting rapid range expansion. India showed highest haplotype richness (h = 4), followed by China (h = 3) and Japan (h = 2). Overall haplotype diversity was moderate (0.205), whereas nucleotide diversity was low (0.00096). AMOVA revealed greater variation within countries (52.82%) than among them (47.18%), with low genetic differentiation (FST = 0.0000) and evidence of gene flow. Neutrality tests (Tajima’s D = -2.149**, Fu and Li’s F = -5.822**, Fu’s Fs = -4.7031**) and a unimodal mismatch distribution indicated population expansion, whereas Indian populations showed signs of genetic drift (Tajima’s D = 0.763). This is the first comprehensive analysis of <i>A. nerii</i> population genetics in India, providing critical insights into diversity, structure, and demographic history. Understanding the genetic diversity, expansion, and gene flow of <i>A. nerii</i> populations provides crucial insights for designing targeted and sustainable pest management strategies to protect <i>G. sylvestre</i> cultivation in India.</p>