Background: There has been a large body of research on obesity and the risk of prostate cancer (PCa) that has been published recently. However, the epidemiological evidence for such an association has not been consistent. This may be attributed to the nature of case-control and retrospective studies, which generally are more prone to biases. Therefore, we conducted a systematic review of prospective cohort studies to assess the association between obesity and the risk of PCa incidence and death. Methods: A search of the PubMed database and references of published studies (from inception until March 2013) was conducted. Twenty-three eligible studies were identified and included in the systematic review. Results: The evidence from the prospective cohort studies linking obesity with PCa incidence has not been consistent. However, cumulative data is compelling for a strong positive association between obesity and fatal PCa. Conclusions: Obesity is a significant diet-related risk factor for fatal PCa. Further well-constructed, large cohort studies on the potential association between obesity and PCa, as well as on underlying mechanisms, are needed.