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