The tumor suppressor gene PTEN (phosphatase and tensin homologue deleted on chromosome 10) is frequently mutated or deleted in various human cancers. PTEN localizes predominantly to the cytoplasm and functions as a lipid phosphatase, thereby negatively regulating the phosphatidylinositol 3-kinase-AKT signaling pathway. PTEN can also localize to the nucleus, where it binds and regulates p53 protein level and transcription activity. However, the precise function of nuclear PTEN and the factors that control PTEN nuclear localization are still largely unknown. In this study, we identified oxidative stress as one of the physiological stimuli that regulate the accumulation of nuclear PTEN. Specifically, oxidative stress inhibits PTEN nuclear export, a process depending on phosphorylation of its amino acid residue Ser-380. Nuclear PTEN, independent of its phosphatase activity, leads to p53-mediated G(1) growth arrest, cell death, and reduction of reactive oxygen species production. Using xenografts propagated from human prostate cancer cell lines, we reveal that nuclear PTEN is sufficient to reduce tumor progression in vivo in a p53-dependent manner. The data outlined in this study suggest a unique role of nuclear PTEN to arrest and protect cells upon oxidative damage and to regulate tumorigenesis. Since tumor cells are constantly exposed to oxidative stress, our study elucidates the cooperative roles of nuclear PTEN with p53 in tumor suppression.