Abstract Targeting the PI3K pathway is a promising strategy for treating prostate cancers with PTEN-loss. However, current anti-PI3K therapies fail to show long lasting in vivo effects. We find that not only the PI3Kα- and PI3kβ-isoforms, but also PI3Kδ, are associated with the epithelial–mesenchymal transition (EMT), a critical process distinguishing indolent from aggressive prostate cancer. This suggests that cotargeting PI3Kα/β/δ could preempt the rebound activation of the parallel pathways induced by α- or β-isoform–selective inhibitor and prevent EMT. Indeed, BAY1082439, a new selective PI3Kα/β/δ inhibitor, is highly effective in vivo in inhibiting Pten-null prostate cancer growth and preventing EMT in the mutant Pten/Kras metastatic model. The anti-PI3Kδ property of BAY1082439 further blocks B-cell infiltration and lymphotoxin release, which are tumor microenvironment factors that promote castration-resistant growth. Together, our data suggest a new approach for the treatment of prostate cancer by targeting both tumor cells and tumor microenvironment with PI3Kα/β/δ inhibitor. Mol Cancer Ther; 17(10); 2091–9. ©2018 AACR.