An absolute or functional deficit in β-cell mass is a key factor in the pathogenesis of diabetes. We model obesity-driven β-cell mass expansion by studying the diabetes-resistant C57BL/6-Leptinob/ob mouse. We previously reported that cholecystokinin (Cck)wasthe most up-regulatedgenein obese pancreatic islets. We now show that islet cholecystokinin (CCK) is up-regulated 500-fold by obesity and expressed in both α- and β-cells. We bred a null Cck allele into the C57BL/6-Leptinob/ob background and investigated β-cell mass and metabolic parameters of Cck-deficient obese mice. Loss of CCK resulted in decreased islet size and reduced β-cell mass through increased β-cell death. CCK deficiency and decreased β-cell mass exacerbated fasting hyperglycemia and reduced hyperinsulinemia. We further investigated whether CCK can directly affect β-cell death in cell culture and isolated islets. CCK was able to directly reduce cytokine- and endoplasmic reticulum stress-induced cell death. In summary, CCK is up-regulated by islet cells during obesity and functions as a paracrine or autocrine factor to increase β-cell survival and expand β-cell mass to compensate for obesity-induced insulin resistance.