Galectin-3, a beta-galactoside-binding protein, has been implicated in a variety of biological functions including cell proliferation, apoptosis, angiogenesis, tumor progression, and metastasis. The present study was undertaken to understand the role of galectin-3 in the progression of prostate cancer. Immunohistochemical analysis of galectin-3 expression revealed that galectin-3 was cleaved during the progression of prostate cancer. Galectin-3 knockdown by small interfering RNA (siRNA) was associated with reduced cell migration, invasion, cell proliferation, anchorage-independent colony formation, and tumor growth in the prostates of nude mice. Galectin-3 knockdown in human prostate cancer PC3 cells led to cell-cycle arrest at G(1) phase, up-regulation of nuclear p21, and hypophosphorylation of the retinoblastoma tumor suppressor protein (pRb), with no effect on cyclin D1, cyclin E, cyclin-dependent kinases (CDK2 and CDK4), and p27 protein expression levels. The data obtained here implicate galectin-3 in prostate cancer progression and suggest that galectin-3 may serve as both a diagnostic marker and therapeutic target for future disease treatments.