© 2019 by the American Diabetes Association. OBJECTIVE Type 2 diabetes (T2D) is a public health threat. Prediabetes represents a window of opportunity for intervention to prevent T2D. Men with T2D and prediabetes often have low testosterone. Since testosterone improves glycemic control in T2D, we investigated whether testosterone therapy (TTh) in men with hypogonadism and prediabetes prevents progression to T2D. RESEARCH DESIGN AND METHODS Three hundred and sixteen men with prediabetes (defined as HbA1c 5.7–6.4%) and total testosterone levels £12.1 nmol/L combined with symptoms of hypogonadism were analyzed. Two hundred and twenty-nine men received parenteral testosterone undecanoate (T-group), and 87 men with hypogonadism served as untreated control subjects. Metabolic and anthropometric parameters were measured twice yearly for 8 years. RESULTS HbA1c decreased by 0.39 6 0.03% (P < 0.0001) in the T-group and increased by 0.63 6 0.1% (P < 0.0001) in the untreated group. In the T-group, 90% achieved normal glucose regulation (HbA1c <5.7%). In the untreated group, 40.2% progressed to T2D (HbA1c >6.5%). TTh was also associated with significant improvements in fasting glucose, triglyceride:HDL ratio, triglyceride-glucose index, lipid accumulation product, total cholesterol, LDL, HDL, non-HDL, triglycerides, and Aging Males’ Symptoms (AMS) scale. Significant deterioration in all these parameters was seen in the untreated group. Mortality was 7.4% in the T-group and 16.1% in the untreated group (P < 0.05). The incidence of nonfatal myocardial infarction was 0.4% in the T-group and 5.7% in the untreated group (P < 0.005). CONCLUSIONS Long-term TTh completely prevents prediabetes progression to T2D in men with hypogonadism and improves glycemia, lipids, and AMS score. TTh holds tremendous potential for the large and growing population of men with prediabetes and hypogonadism.