Abstract: Plutella xylostella, a major pest of cruciferous vegetables worldwide, has developed resistance to diamide insecticides. Thiotraniliprole, a novel synthetic diamide insecticide, exhibits excellent activity against P. xylostella. In the present study, we aimed to confirm the resistance risk, cross-resistance, and mechanisms of resistance to thiotraniliprole in P. xylostella. After 40 consecutive generations of thiotraniliprole selection, we obtained a thiotraniliprole-resistance P. xylostella strain with a 5141.58-fold resistance ratio (RR) to thiotraniliprole. The overall realized heritability (h²) value of resistance was estimated as 0.9 using threshold trait analysis, indicating that the risk of developing resistance to thiotraniliprole is high in P. xylostella. The thiotraniliprole-resistant (TR) strain showed noticeable cross-resistance to chlorantraniliprole (RR = 44670.05), cyantraniliprole (RR = 7038.58), and tetrachlorantraniliprole (RR = 1506.01), but no cross-resistance to tolfenpyrad, indoxacarb, diafenthiuron, or abamectin compared with the susceptible (S) strain. The enzyme assay data showed that the activities of glutathione-S transferase (GST), carboxylesterase (CarE), and the content of cytochrome P450 monooxygenase (P450s) were significantly higher in the TR strain than in the S strain. Sequencing of the full-length PxRyR cDNA revealed the gene site I4790K in the TR strain with a 100% frequency. This mutation in PxRyR likely underlies the high-level cross-resistance between thiotraniliprole and three other diamide insecticides. These findings provide valuable information for optimizing resistance management strategies to delay thiotraniliprole resistance development and ensure sustainable control of P. xylostella.