Accumulating evidence indicates that patients treated with rosuvastatin have an increased risk of developing new-onset diabetes. However, the underlying mechanism remains unclear. In this study, we administered rosuvastatin (10 mg/kg body weight) to male C57BL/6J mice for 12 weeks and found that oral rosuvastatin dramatically reduced intraperitoneal glucose tolerance. Rosuvastatin-treated mice showed considerably higher serum levels of branched-chain amino acids (BCAAs) than control mice. They also showed dramatically altered expression of BCAA catabolismrelated enzymes in white adipose tissue and skeletal muscle, including downregulated mRNA expression of BCAT2 and protein phosphatase 2Cm (PP2Cm) and upregulated mRNA expression of branched-chain ketoacid dehydrogenase kinase (BCKDK). The levels of BCKD in the skeletal muscle were reduced in rosuvastatin-treated mice, which was associated with lower PP2Cm protein levels and increased BCKDK levels.We also investigated the effects of rosuvastatin and insulin administration on glucose metabolism and BCAA catabolism in C2C12 myoblasts. We observed that incubation with insulin enhanced glucose uptake and facilitated BCAA catabolism in C2C12 cells, which was accompanied by elevated Akt and glycogen synthase kinase 3 β (GSK3β) phosphorylation. These effects of insulin were prevented by co-incubation of the cells with 25 μM rosuvastatin. Moreover, the effects of insulin and rosuvastatin administration on glucose uptake and Akt and GSK3β signaling in C2C12 cells were abolished when PP2Cm was knocked down. Although the relevance of these data, obtained with high doses of rosuvastatin in mice, to therapeutic doses in humans remains to be elucidated, this study highlights a potential mechanism for the diabetogenic effects of rosuvastatin, and suggests that BCAA catabolism could be a pharmacological target for preventing the adverse effects of rosuvastatin.