Men with obesity tend to be insulin resistant and often have low-normal testosterone concentrations. We conducted a clinical trial aimed to evaluate potential therapeutic strategies for low testosterone in men with obesity. We did a 1-year, parallel, randomized, double-blind, placebo-controlled trial, where we evaluated the independent and combined effects of metformin and testosterone in 106 men with obesity, aged 18–50 years, who had low levels of testosterone and no diabetes mellitus. The primary outcome was change in insulin resistance, measured as Homeostasis Model Assessment for Insulin Resistance (HOMA-IR) index. Secondary outcomes included changes in total and free serum testosterone, body composition, metabolic variables, erectile function, and health-related quality of life (HRQoL). In the intention-to-treat analysis, the HOMA-IR index decreased significantly in all active groups compared to placebo (metformin −2.4, 95 % CI −4.1 to −0.8, p = 0.004; testosterone −2.7, 95 % CI −4.3 to −1.1, p = 0.001; combination −3.4, 95 % CI −5.0 to −1.8, p < 0.001). Combination therapy was not superior to testosterone alone in decreasing insulin resistance (−0.7, 95 % CI −2.3 to 0.9, p = 0.383). Only the combination of metformin plus testosterone significantly increased total and free testosterone concentrations, compared to placebo. No significant changes in body composition (except for a higher decrease in fat mass in the metformin and combination group), metabolic variables, erectile function, or HRQoL were found with any treatment. Among men with obesity and low testosterone concentrations, the combination of metformin plus testosterone, metformin only, and testosterone only, compared to placebo, reduced insulin resistance with no evidence of additive benefit. Display omitted •Men with obesity tend to be insulin resistant and often have low-normal testosterone concentrations.•We conducted a randomized, double-blind, placebo-controlled trial of metformin, testosterone, or both, with the endpoint of insulin resistance.•Metformin only, testosterone only, and the combination of metformin plus testosterone reduced insulin resistance with no evidence of additive benefit.