Castration-resistant prostate cancer (CRPC) continues to be a major clinical problem and the mechanisms behind it remain unclear. Thioredoxin domain-containing protein 5 (TXNDC5) is involved in protein folding and chaperone activity, and its overexpression has been reported in multiple malignancies. In the current study, we demonstrated that TXNDC5 is up-regulated following long-term androgen-deprivation treatment (ADT) and is highly overexpressed in CRPC tumors compared with hormone-naive prostate cancer (PCa) cases. Functionally, in vitro and in vivo studies demonstrated that TXNDC5 overexpression promotes the growth of both androgen-dependent and castration-resistant PCa xenografts. Mechanistically, TXNDC5 directly interacts with the AR protein to increase its stability and thus enhances its transcriptional activity. TXDNC5-mediated CRPC growth can be fully abolished by AR inhibition, suggesting TXDNC5 up-regulation as an escape pathway for aberrant AR re-activation and CRPC growth in the milieu of low androgen. Indeed, we found that TXNDC5 is increased by ADT-induced hypoxia through HIF-1α in an miR-200b-dependent manner. Overall, we defined an important role of TXNDC5 in CRPC and further investigations are needed to screen TXNDC5 antagonists as a novel therapeutic approaches to treat PCa patients with CRPC.