The term "flavonoids" is created for a large class of polyphenolic compounds featuring two aromatic rings (A and B) and a central heterocyclic ring C. More than 4,000 flavonoids have already been obtained throughout the plant kingdom. An inverse correlation between the intake of dietary flavonoids and the incidence of prostate cancer can be generally well-observed from several epidemiological studies worldwide, which encouraged several research groups to examine both in vitro and in vivo the potential of whole extracts of flavonoidenriched dietary plants and purified flavonoids in preventing and treating prostate cancer. Nine subclasses of naturally occurring dietary flavonoids have so far been explored and identified to possess clinical potential in preventing and treating prostate cancer. We review herein the antiprostate cancer potential (including in vitro potency, in vivo efficacy, and clinical studies), structure-activity relationships, and the mechanism of actions of the nine subclasses of naturally occurring flavonoids.