In phase I/II clinical trials, Z-endoxifen demonstrated substantial oral bioavailability and promising antitumor activity in endocrine-refractory estrogen-receptor positive breast cancer (ER+ BC) and other solid tumors, with plasma concentrations reportedly as high as 5 �M. Therefore, we explored the potential mechanisms of Z-endoxifen antitumor activity that extends beyond ERα inhibition. In estrogen unstimulated aromatase-expressing ER+/human epidermal growth factor 2 receptor negative (HER2-) MCF7AC1 BC cells, Z-endoxifen at 5 �M, but not ER�-targeting 0.01 and 0.1 �M concentrations inhibited growth and induced apoptosis, suggesting an ERα-independent effect. Utilizing an unbiased mass spectrometry approach, we explored Z-endoxifen effects on other signaling pathways. Z-endoxifen at 5 µM profoundly altered the phosphoproteome with minimal impact on total proteome. Computational analysis revealed Protein kinase C beta (PKC�) and AKT1 as the prevalent upstream kinases for Z-endoxifen-downregulated protein phosphorylations. Notably, in ER+/HER2- BC models, Z-endoxifen at 5 �M attenuated AKTSer473 and AKT substrates in vitro in the presence of insulin and PKC agonist PMA and in vivo. Further, Z-endoxifen inhibited PKC�1 kinase activity compared to other PKC isoforms in vitro and bound to PKCβ1. While PMA stimulated PKC�1Ser661 phosphorylation correlated with AKTSer473 and AKT substrate phosphorylation, Z-endoxifen at 5 �M uniquely blocked these effects and induced PKCβ1 protein degradation. siRNA-mediated PKC�1 knockdown attenuated AKTSer473 phosphorylation, suggesting PKCβ1-mediated suppression of AKT signaling by Z-endoxifen. Further, Z-endoxifen at 5 �M replicates the pan-AKT inhibitor MK-2206 effects on apoptosis. These findings implicate PKCβ1 as a novel Z-endoxifen substrate responsible for suppressing AKT signaling and inducing apoptosis in breast cancer.