We previously demonstrated that the mTORC1/S6K1 pathway is activated by insulin and nutrient overload (e.g. amino acids [AA]), which leads to the inhibition of the PI3K/Akt pathway via the inhibitory serine phosphorylation of IRS-1, notably on serine 1101 (Ser1101). However, even in the absence of AA, insulin can still promote IRS-1 Ser1101 phosphorylation by other kinases that remain to be fully characterized. Here, we describe a new negative regulator of IRS-1, the p90 ribosomal S6 kinase (RSK). Computational analyses revealed that Ser1101 within IRS-1 falls into the consensus motif of RSK. Moreover, recombinant RSK phosphorylated IRS-1 C-terminal fragment on Ser1101 which was prevented by mutations of this site or when a kinase-inactive mutant of RSK was used. Using antibodies directed toward the phosphorylation sites located in the activation segment of RSK (S221 or S380), we found that insulin activates RSK in L6 myocytes in the absence of AA overload. Inhibition of RSK using either the pharmacological inhibitor BI-D1870 or after adenoviral expression of a dominant negative RSK1 mutant (RSK1-DN) showed that RSK selectively phosphorylates IRS-1 on Ser1101. Accordingly, expression of the RSK1-DN mutant in L6 myocytes and FAO hepatic cells improved insulin action on glucose uptake and glucose production, respectively. Furthermore, RSK1 inhibition prevented insulin resistance in L6 myocytes chronically exposed to high glucose and high insulin. These results show that RSK is a novel regulator of insulin signaling and glucose metabolism and a potential mediator of insulin resistance, notably through the negative phosphorylation of IRS-1 on Ser1101.