Objective To assess the effect of long-term strength training on insulin sensitivity, lipid profile, and body composition in overweight and obese subjects. Materials and methods A prospective, randomized, interventional study of 16 overweight or obese subjects aged 18–35 years who were investigated before and at the end of 8 weeks of strength training. The experimental group (n = 8) followed a strength training program consisting of 4 sessions per week at 50% to 80% of repetition maximum (RM), estimated through the 1RM test. The control group (n = 8) did not perform the training program. Glucose, insulin, total cholesterol, triglycerides, HDL-C, VLDL-C, and LDL-C levels and arterial index were determined. Insulin sensitivity was measured by calculating HOMA-IR (Homeostatic Model Assessment-Insulin Resistance). Indicators of body composition included weight, height, waist circumference, body fat, fat weight, muscle mass, somatotype chart and distance. Results At the end of intervention, the experimental group showed decreased insulin sensitivity (3.5 ± 0.9 vs. 2.9 ± 1.2; p = 0.04), LDL-C (106.9 ± 20.8 vs. 95.5 ± 14.2; p = 0.03) and arterial index (4.0 ± 0.6 vs. 3.5 ± 0.5; p = 0.01) values and increased HDL-C levels (43.7 ± 8.8 vs. 46.9 ± 5.6; p = 0.04), while the control group remained stable. There were no significant differences between the groups regarding body composition, somatotype chart or distance after training. Conclusions In overweight and obese subjects, strength training for eight weeks improved insulin sensitivity and lipid profile without altering body composition.