This review provides the epidemiologic and research evidences documenting the effects of coffee consumption on type 2 diabetes mellitus (T2DM). We summarize the literature concerning the effects of coffee consumption on different mechanistic factors involving in pathogenesis of T2DM, such as glucose tolerance, insulin sensitivity, insulin resistance, glucose-6-phosphatase, intestinal glucose absorption, antioxidant activity, inflammatory biomarkers, nuclear factor-κB inhibition, glucose uptake, glucose homeostasis, glucose metabolism, and insulin secretion. These factors play a crucial role in influencing the normal levels of glucose in blood. Overall, the experimental and epidemiologic evidences presented here elucidate the protective effects of coffee consumption on T2DM, involving multiple preventive mechanisms. Despite the firm evidences available through a growing literature base, it is still uncertain whether the use of coffee should be recommended to patients with diabetes and/or any patient who might be at the risk of T2DM as a supplementary therapy to prevent further progression of T2DM.