The androgen receptor (AR) plays a critical role in the development and the progression of prostate cancer. Alterations in the expression of AR coregulators lead to AR hypersensitivity, which is one of the mechanisms underlying the progression of prostate cancer into a castrate-resistant state. Octamer transcription factor 1 (Oct1) is a ubiquitous member of the POU-homeodomain family that functions as a coregulator of AR. In our study, the contribution of Oct1 to prostate cancer development was examined. Immunocytochemistry analysis showed that Oct1 is expressed in the nuclei of LNCaP cells. siRNA-mediated silencing of Oct1 expression inhibited LNCaP cell proliferation. Immunohistochemical analysis of Oct1 expression in tumor specimens obtained from 102 patients with prostate cancer showed a positive correlation of Oct1 immunoreactivity with a high Gleason score and AR immunoreactivity (p= 0.0042 andp< 0.0001, respectively). Moreover, patients with high immunoreactivity of Oct1 showed a low cancer-specific survival rate, and those patients with high immunoreactivities of both Oct1 and AR exhibited poorer cancer-specific prognosis. Multivariate hazard analysis revealed a significant correlation between high Oct1 immunoreactivity and poor cancer-specific survival (p= 0.012). These results demonstrate that Oct1 can be a prognostic factor in prostate cancer as a coregulator of AR and may lead to the development of a new therapeutic intervention for prostate cancer.