Abstract Local androgen synthesis in prostate cancer (PC) may contribute to the development of castration-resistant PC (CRPC), but pathways controlling intratumoral steroidogenic enzyme expression in PC are unknown. We investigated the effects of activin, a factor involved in the regulation of PC growth and steroidogenic enzyme expression in other steroidogenic tissues, on intratumoral steroidogenesis in PC. Activin A effects and regulation of the activin-signaling pathway molecules were studied in the PC cell lines LNCaP, VCaP, and PC-3 and in 13 individual PC xenograft models. Also, expression levels of inhibin βA- and βB-subunits (INHBA and INHBB) and of the activin antagonist follistatin were quantitated in patient PC tissues. Activin A induced the expression and enzyme activity of 17β-hydroxysteroid dehydrogenase enzyme AKR1C3 in LNCaP and VCaP cells. Inhibition of endogenous activin A action in the PC-3 cell line decreased AKR1C3 levels and consequently testosterone synthesis. In return, androgens suppressed INHBA expression in both VCaP cells and the PC xenograft models. The antiproliferative effects of activin A were opposed by physiological concentrations of androstenedione in LNCaP cells. In patient PC tissues, expression levels of INHBA were increased in CRPC samples and correlated with AKR1C3 levels. Moreover, a high ratio of activin subunits to follistatin was associated with a worse metastasis-free survival in patients. In conclusion, activin A is controlled by androgens in PC models and regulates local androgen production. Activin A thus seems to mediate (residual) intratumoral androgen levels and could form a novel therapeutic target in CRPC.