Prostate cancer prognosis may therefore be improved by maintaining healthy weight through diet and physical activity. This systematic review looked at the effect of diet and exercise interventions on body weight among men treated for prostate cancer. MEDLINE, EMBASE, CINAHL, and Cochrane Library databases were searched from the earliest record to August 2013. Randomized controlled trials of diet and exercise interventions in prostate cancer patients that reported body weight or body composition changes were included. A total of 20 trials were included in the review. Because of the heterogeneity of intervention components, a narrative review was conducted. Interventions were categorized as diet (n = 6), exercise (n = 8), or a combination of both diet and exercise (n = 6). The sample size ranged from 8 to 155 and the duration from 3 wk to 4 yr. Four diet interventions and 1 combined diet and exercise intervention achieved significant weight loss with mean values ranging from 0.8 kg to 6.1 kg (median 4.5 kg). Exercise alone did not lead to weight loss, though most of these trials aimed to increase fitness and quality of life rather than decrease body weight. Diet intervention, alone or in combination with exercise, can lead to weight loss in men treated for prostate cancer.