Insulin promotes the cardiovascular protective functions of the endothelium including NO production by endothelial NO synthase (eNOS), which it stimulates via Akt kinase which phosphorylates eNOS Ser1179. C-reactive protein (CRP) is an acute-phase reactant that is positively correlated with cardiovascular disease risk in patients with type 2 diabetes. We previously showed that CRP inhibits eNOS activation by insulin by blunting Ser1179 phosphorylation. We now elucidate the underlying molecular mechanisms. We first show in mice that CRP inhibits insulin-induced eNOS phosphorylation, indicating that these processes are operative in vivo. In endothelial cells we find that CRP attenuates insulin-induced Akt phosphorylation, and CRP antagonism of eNOS is negated by expression of constitutively active Akt; the inhibitory effect of CRP on Akt is also observed in vivo. A requirement for the IgG receptor FcRIIB was demonstrated in vitro using blocking antibody, and reconstitution experiments with wild-type and mutant FcRIIB in NIH3T3^IR cells revealed that these processes require the ITIM (immunoreceptor tyrosine-based inhibition motif) of the receptor. Furthermore, we find that endothelium express SHIP-1 (Src homology 2 domain–containing inositol 5'-phosphatase 1), that CRP induces SHIP-1 stimulatory phosphorylation in endothelium in culture and in vivo, and that SHIP-1 knockdown by small interfering RNA prevents CRP antagonism of insulin-induced eNOS activation. Thus, CRP inhibits eNOS stimulation by insulin via FcRIIB and its ITIM, SHIP-1 activation, and resulting blunted activation of Akt. These findings provide mechanistic linkage among CRP, impaired insulin signaling in endothelium, and greater cardiovascular disease risk in type 2 diabetes.