Eating a “Westernized” diet high in fat and sugar leads to weight gain and numerous health problems, including the development of type 2 diabetes mellitus (T2DM). Rodent studies have shown that resveratrol supplementation reduces blood glucose levels, preserves β-cells in islets of Langerhans, and improves insulin action. Although rodent models are helpful for understanding β-cell biology and certain aspects of T2DM pathology, they fail to reproduce the complexity of the human disease as well as that of nonhuman primates. Rhesus monkeys were fed a standard diet (SD), or a high-fat/high-sugar diet in combination with either placebo (HFS) or resveratrol (HFS+Resv) for 24 months, and pancreata were examined before overt dysglycemia occurred. Increased glucose-stimulated insulin secretion and insulin resistance occurred in both HFS and HFS+Resv diets compared with SD. Although islet size was unaffected, there was a significant decrease in β-cells and an increase in α-cells containing glucagon and glucagon-like peptide 1 with HFS diets. Islets from HFS+Resv monkeys were morphologically similar to SD. HFS diets also resulted in decreased expression of essential β-cell transcription factors forkhead box O1 (FOXO1), NKX6–1, NKX2–2, and PDX1, which did not occur with resveratrol supplementation. Similar changes were observed in human islets where the effects of resveratrol were mediated through Sirtuin 1. These findings have implications for the management of humans with insulin resistance, prediabetes, and diabetes.