There is a need to characterize promising dietary agents for chemoprevention and therapy of prostate cancer (PCa). We examined the anticancer effect of α-mangostin, derived from the mangosteen fruit, in human PCa cells and its role in targeting cell cycle-related proteins involved in prostate carcinogenesis. Using an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, we found that α-mangostin significantly decreases PCa cell viability in a dose-dependent manner. Further analysis using flow cytometry identified cell cycle arrest along with apoptosis. To establish a more precise mechanism of action, we performed a cell free biochemical kinase assay against multiple cyclins/cyclin-dependent kinases (CDKs) involved in cell cycle progression; the most significant inhibition in the cell free-based assays was CDK4, a critical component of the G1 phase. Through molecular modeling, we evaluated α-mangostin against the adenosine triphosphate-binding pocket of CDK4 and propose three possible orientations that may result in CDK4 inhibition. We then performed an in vivo animal study to evaluate the ability of α-mangostin to suppress tumor growth. Athymic nude mice were implanted with 22Rv1 cells and treated with vehicle or α-mangostin (100 mg/kg) by oral gavage. At the conclusion of the study, mice in the control cohort had a tumor volume of 1190 mm(3), while the treatment group had a tumor volume of 410 mm(3) (P < 0.01). The ability of α-mangostin to inhibit PCa in vitro and in vivo suggests α-mangostin may be a novel agent for the management of PCa.