Increased sugar consumption, particularly fructose, in the form of sweetened beverages and sweeteners in our diet adversely affects metabolic health. Because these effects are associated with features of the metabolic syndrome in humans, the direct effect of fructose on pancreatic islet function is unknown. We therefore examined the islet phenotype of mice fed excess fructose. Fructose-fed mice exhibited fasting hyperglycemia and glucose intolerance, but not hyperinsulinemia, dyslipidemia, or hyperuricemia. Islet function was impaired, with decreased glucose-stimulated insulin secretion and increased glucagon secretion, High fructose consumption led to alpha cell proliferation and upregulation of the fructose transporter GLUT5, which was localized only in alpha cells. Our studies demonstrate that excess fructose consumption contributes to hyperglycemia by affecting both beta and alpha cells of islets in mice.