The citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae), is regarded as one of the most important citrus pests in many countries, such as Japan, Spain, and China. In this study, the mitochondrial cytochrome oxidase subunit I gene was used to analyze genetic diversity and population structure of 15 P. citri populations collected from three citrus (Citrus spp.) planting regions of China. Our analysis found that these populations had relatively low genetic diversities. Bayesian tree and haplotype network showed that the 22 haplotypes of P. citri formed two lineages with low Bayesian posterior probabilities (0.55 and 0.63), and haplotypes in a sample were scattered throughout the Bayesian tree, indicating that there may be no significant genealogical structure among populations. However, when Yuxi and Danjiangkou populations (both without H7 haplotype) were excluded from the analysis, analysis of molecular variance found a weak, but significant, geographic structuring. Mantel test indicated a significant and positive correlation between genetic and geographical distances, reflecting certain degree of isolation by distance. The genetic differentiation based on pairwise F(ST) was not significant between most populations, and some F(ST) were even negative, indicating remarkable gene flow among these populations. The weak population structure of P. citri in this study was probably influenced by high gene flow between some populations due to long-distance dispersal of this species, which may be largely relied on the movement of plants between populations. The mismatch distribution analysis showed that no signal of population growth, but the Fu's F(S) value was significantly negative for total populations and the star-like shape of haplotype network, suggesting a history of population expansion of P. citri in China.