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Cytochrome P450 monooxygenases (P450s) and UDP-glycosyltransferases (UGTs) are involved in the evolution of insecticide resistance. <i>Leptinotarsa decemlineata</i> (Say), the Colorado potato beetle (CPB), is a notorious insect that has developed resistance to various insecticides including neonicotinoids. This study investigated whether the differentially expressed P450 genes <i>CYP9Z140</i> and <i>CYP9AY1</i> and UGT gene <i>UGT321AP1</i>, found in our transcriptome results, conferred resistance to thiamethoxam in <i>L. decemlineata</i>. Resistance monitoring showed that the sampled field populations of <i>L. decemlineata</i> adults collected from Urumqi City and Qapqal, Jimsar, and Mulei Counties of Xinjiang in 2021–2023 developed low levels of resistance to thiamethoxam with resistance ratios ranging from 6.66- to 9.52-fold. Expression analyses indicated that <i>CYP9Z140</i>, <i>CYP9AY1</i>, and <i>UGT321AP1</i> were significantly upregulated in thiamethoxam-resistant populations compared with susceptible populations. The expression of all three genes also increased significantly after thiamethoxam treatment compared with the control. Spatiotemporal expression patterns showed that the highest expression of <i>CYP9Z140</i> and <i>CYP9AY1</i> occurred in pupae and the midgut, whereas <i>UGT321AP1</i> was highly expressed in adults and Malpighian tubules. Knocking down all three genes individually or simultaneously using RNA interference increased the sensitivity of adult <i>L. decemlineata</i> to thiamethoxam. These results suggest that overexpression of <i>CYP9Z140</i>, <i>CYP9AY1,</i> and <i>UGT321AP1</i> contributes to the development of thiamethoxam resistance in <i>L. decemlineata</i> and provides a scientific basis for improving new resistance management of CPB.