G protein-coupled receptors (GPCRs) are considered the largest and most variable family of transmembrane receptors regulating physiological processes such as toxicological responses and insecticide resistance development. The present study investigated the responses of Methuselah (Mth), belonging to GPCR family B in the Diamondback Moth (DBM), Plutella xylostella, to chlorantraniliprole (CAP). Genome-wide identification and phylogenetic analysis of Pxmth genes revealed their evolutionary relationships and functional classifications. Expression profiling demonstrated significant overexpression of Pxmth2 in the CAP-resistant strain. Additionally, the tertiary and secondary structures of Pxmth2 were characterized, providing insights into its functional role. Silencing Pxmth2 via RNA interference (RNAi) reduced resistance of DBM to CAP and suppressed downstream stress-associated genes (CYP6B6, CYP6B7, CYP6BF1), increasing susceptibility to the insecticide. The function of Pxmth2 was further explored using a transgenic line of Drosophila melanogaster engineered to overexpress the gene; flies overexpressing Pxmth2 exhibited a significantly increased resistance to CAP compared to controls. These findings indicate that Pxmth2 contributes to CAP resistance in DBM and highlights potential molecular targets for improving pest management strategies.