Obolodiplosis robiniae is native to North America and is an important introduced insect pest that forms leaf margin roll galls on species of genus Robinia (Fabaceae) in China. It was first detected in China in 2004, but subsequently spread and provoked local outbreaks. An analysis of a 676-bp sequence of the mitochondrial DNA cytochrome oxidase subunit I was conducted in 560 individuals from 28 populations, in order to (1) assess population genetic structuring and (2) explore possible explanations for the rapid spread and invasion success of O. robiniae. Yet, only four haplotypes were identified and the nucleotide diversity was low (π = 0.00005) and among the 560 specimens studied, only ten showed haplotypic variation involving no more than three substitutions. The result showed a low degree of genetic diversity among populations of the successful invasive gall midge, which suggested that the pest experienced a severe genetic bottleneck and a loss of genetic diversity after its introduction. The successful establishment and spread of O. robiniae in China is attributed to the wide distribution of its host plant, thus allowing ample opportunities for gene flow in the pest species, and to the advantageous life history characteristics of O. robiniae.