Prostate cancer is the most commonly diagnosed non-skin cancer in men in the United States. Among environmental factors, diet may play a particularly important role in its incidence, progression, and clinical outcome. This article reviews the findings of eight observational studies and 17 intervention or laboratory trials on the effect of plant-based diets and plant nutrients on both the progression and clinical outcome of prostate cancer as well as additional studies examining mechanisms that may explain dietary effects. While additional long-term therapeutic clinical trials are needed to further elucidate the role of diet, these early investigations suggest that a recommendation for individual patients to shift their diets toward plant foods may serve as an important component of the tertiary treatment of prostate cancer.