Despite the development of second-generation antiandrogens, acquired resistance to hormone therapy remains a major challenge in treating advanced prostate cancer. We find that cancer-associated fibroblasts (CAFs) can promote antiandrogen resistance in mouse models and in prostate organoid cultures. We identify neuregulin 1 (NRG1) in CAF supernatant, which promotes resistance in tumor cells through activation of HER3. Pharmacological blockade of the NRG1/HER3 axis using clinical-grade blocking antibodies re-sensitizes tumors to hormone deprivation in vitro and in vivo. Furthermore, patients with castration-resistant prostate cancer with increased tumor NRG1 activity have an inferior response to second-generation antiandrogen therapy. This work reveals a paracrine mechanism of antiandrogen resistance in prostate cancer amenable to clinical testing using available targeted therapies. Display omitted •CAF-derived NRG1 confers antiandrogen resistance in prostate cancer•Pharmacological blockade of the NRG1-HER3 axis induces tumor regression•NRG1 is upregulated in prostate stroma in patients after androgen deprivation therapy•High NRG1 activity and reduced sensitivity to second-generation antiandrogen therapy Zhang et al. find that cancer-associated fibroblasts promote antiandrogen resistance in prostate cancer by secreting NRG1 to activate HER3 signaling in prostate cancer cells. Blockade of the NRG1/HER3 axis can re-sensitize prostate cancer models to antiandrogen therapy.