Abstract Sublethal concentrations of insecticides are commonly encountered in agricultural environments, particularly by pests such as the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), which primarily infests host plants during the larval stage. Sublethal concentrations of insecticides can elicit a wide range of effects; therefore, it is important to consider the impact of thiamethoxam, a registered control insecticide for B. dorsalis. This study investigated the sublethal effects of thiamethoxam on B. dorsalis. Exposure to sublethal concentrations of thiamethoxam (LC10 and LC30) significantly increased fecundity, with egg production rising by 37.6% and 89.4%, and in egg production of its offspring by 8.5% and 21.9%. There was also a significant reduction in longevity of females by 3.1% and 5.3%, and of males by 2.3% and 6.8%. Furthermore, at 0, 3, and 6 d after 24 h of treatment, sublethal concentrations of thiamethoxam significantly upregulated the expression of vitellogenin (Vg) and vitellogenin receptor (VgR) and significantly increased the levels of Vg, VgR, juvenile hormone (JH), and ecdysone. These findings suggest that the mechanism underlying the enhancement of fecundity by sublethal concentrations of thiamethoxam may involve increased levels of JH and ecdysone, which regulate Vg synthesis, thereby stimulating reproductive output, with JH likely playing a dominant role. This study is the first to report the stimulatory effect of sublethal thiamethoxam concentrations on B. dorsalis fecundity and provides preliminary insights into the associated mechanisms. The results offer valuable references for understanding the sublethal effects of thiamethoxam on B. dorsalis and optimizing integrated pest management strategies.