Background: Androgen targeted therapy remains the mainstay for advanced prostate cancer (PC). Second-generation androgen receptor (AR) antagonist, enzalutamide (ENZ), re-targets persistent AR activity in castrate-resistant (CR) PC tumors, and is approved for CRPC. Despite initial clinical activity, acquired resistance to ENZ arises rapidly and most patients succumb to PC. Mechanisms underlying resistance to ENZ are incompletely understood. Prior work has established Stat5 as a potent inducer of PC growth. Here, we investigated the significance of Jak2-Stat5 signaling in ENZ-resistant growth of PC. Methods: Levels of Jak2 and Stat5 activation in PC cells, tumors and patient samples were evaluated by immunohistochemistry, 3D tumor explant cultures and western blotting. Jak2 and Stat5 were inhibited by lentiviral (expression of) shRNA or pharmacologically. Levels of mRNA were assessed by QPCR and gene expression profiling. Results: ENZ induced a robust increase in Stat5 activation in PC cells in vitro, in xenograft tumors in vivo and in patient-derived PCs during ENZ treatment. Mechanistically, ENZ activation of Stat5 involves a positive feed-forward mechanism where ENZ-liganded AR induces rapid and sustained Jak2 phosphorylation in PC cells through a process involving Jak2-specific phosphatases. This results in a formation of a positive feed-forward loop in PC where activated Stat5 induces Jak2 mRNA and protein levels in PC. We showed that active Stat5 increased viability of PC cells during ENZ treatment and, at the same time, inhibition of Stat5 as a second-line treatment induced excessive death of PC cells surviving ENZ treatment. Importantly, pharmacological Stat5 blockade inhibited CR growth of PC xenograft tumors after ENZ resistance developed. Conclusions: Collectively, this work introduces a novel concept for a pivotal role of Jak2-Stat5 signaling in mediating resistance of PC to ENZ. Pharmacological Jak2-Stat5 inhibition may provide efficacious therapy in advanced PC in combination with ENZ or after ENZ fails.