Parasympathetic stimulation of pancreatic islets augments glucose-stimulated insulin secretion by inducing inositol trisphosphate receptor (IP₃R)–mediated calcium ion (Ca²⁺) release. Ankyrin-B binds to the IP₃R and is enriched in pancreatic beta cells. We found that ankyrin-B–deficient islets displayed impaired potentiation of insulin secretion by the muscarinic agonist carbachol, blunted carbachol-mediated intracellular Ca²⁺ release, and reduced the abundance of IP₃R. Ankyrin-B–haploinsufficient mice exhibited hyperglycemia after oral ingestion but not after intraperitoneal injection of glucose, consistent with impaired parasympathetic potentiation of glucose-stimulated insulin secretion. The R1788W mutation of ankyrin-B impaired its function in pancreatic islets and is associated with type 2 diabetes in Caucasians and Hispanics. Thus, defective glycemic regulation through loss of ankyrin-B–dependent stabilization of IP₃R is a potential risk factor for type 2 diabetes.