Androgen antagonists or androgen deprivation is a primary therapeutic modality for the treatment of prostate cancer. Invariably, however, the disease becomes progressive and unresponsive to androgen ablation therapy (hormone refractory). The molecular mechanisms by which the androgen antagonists inhibit prostate cancer proliferation are not fully defined. In this report, we demonstrate that SIRT1, a nicotinamide adenosine dinucleotide-dependent histone deacetylase linked to the regulation of longevity, is required for androgen antagonist-mediated transcriptional repression and growth suppression. Androgen antagonist-bound androgen receptor (AR) recruits SIRT1 and NCoR to AR-responsive promoters and deacetylates histone H3 locally at the PSA promoter. Furthermore, SIRT1 down-regulation by siRNA or by pharmacological means increased the sensitivity of androgen-responsive genes to androgen stimulation, enhanced the sensitivity of prostate cancer cell proliferative responses to androgens, and decreased the sensitivity of prostate cancer cells to androgen antagonists. In this study, we demonstrate the ligand-dependent recruitment of a class III HDAC into a co-repressor transcriptional complex, and a necessary functional role for a class III HDAC as a transcriptional co-repressor in AR antagonist-induced transcriptional repression. Collectively, these findings identify SIRT1 as a co-repressor of AR and elucidate a new molecular pathway relevant to prostate cancer growth and approaches to therapy.