To perform a systematic review and network meta-analysis to investigate the most effective intervention for improving body composition outcomes in prostate cancer patients during or after treatment. A systematic search was undertaken in multiple databases from inception to December 2020. Randomized clinical trials examining the effects of exercise/physical activity and/or nutrition interventions on body composition and body weight measures in prostate cancer patients were included. The primary endpoints were both whole-body and regional fat mass and lean mass measures, with body weight and BMI as secondary outcomes. A frequentist random-effects network meta-analysis was undertaken to examine the clustering effect of intervention modalities or control groups on the outcomes of interest. The study protocol is publicly available on PROSPERO (CRD42020202339). Fifty articles describing 47 trials (n = 3207) were included. Resistance training and combined resistance and aerobic exercise were the most effective interventions to reduce body fat percentage (-0.9%; 95% confidence interval [CI], -1.4% to -0.3%) and fat mass (-0.5 kg; 95% CI, -0.9 to -0.1 kg), respectively. For whole-body and regional lean mass, combined resistance and aerobic exercise + healthy diet (0.6 kg; 95% CI, 0.1 to 1.0 kg) and resistance training alone (0.7 kg, 95% CI: 0.4 to 1.0 kg) were the best intervention, respectively. A low-fat diet was the most effective for reducing body weight immediately after or at follow-up, while no intervention promoted significant reductions in BMI. These results indicate that a resistance-based exercise program alone or combined with a general healthy diet are the most effective interventions for improving overall body composition in men with prostate cancer.