Androgen and the androgen receptor (AR) have important roles in prostate cancer (PCa) development, and androgen ablation has been the main therapeutic option for the treatment of PCa. However, the transition mechanism from androgen-dependent to -independent PCa after androgen depletion remains unclear. We investigated the distinct roles of small leucine zipper protein (sLZIP) in proliferation of androgen-dependent and -independent PCa cells. Cyclin D3 is known to interact with AR and attenuates the ligand-dependent function of AR in PCa cells. sLZIP regulates the transcription of cyclin D3 by binding directly to the AP-1 region in the cyclin D3 promoter. sLZIP represses AR transcriptional activity by interaction with AR that is phosphorylated by cyclin D3/cyclin-dependent kinase11(p58), leading to the suppression of androgen-dependent proliferation of PCa cells. The expression level of sLZIP is elevated in androgen-independent PCa cells and advanced human prostate tumors. Knockdown of endogenous sLZIP suppresses proliferation of androgen-independent PCa cells. LNCaP cells transformed to androgen-independent PCa cells exhibit increased expressions of sLZIP and cyclin D3. Tumor formation is inhibited in nude mouse xenografts from two androgen-independent PCa cells that are stably transfected with sh-sLZIP. Our findings indicate that sLZIP negatively regulates AR transactivation in androgen-dependent PCa cells and functions as a positive regulator in tumor progression of androgen-independent PCa. sLZIP contributes to the malignant phenotype of PCa and constitutes a novel therapeutic target for human PCa.