Interleukin 24 (IL-24) is a tumor-suppressing protein currently in clinical trials. We previously demonstrated that IL-24 leads to apoptosis in cancer cells through protein kinase A (PKA) activation in human breast cancer cells. To better understand the mechanism by which IL-24 induces apoptosis, we analyzed the role of glycogen synthase kinase-3 beta (GSK3β), a highly conserved serine/threonine kinase in cancer cells and a downstream target of PKA. Our studies show for the first time that GSK3β is inhibited following IL-24 treatment in human prostate cancer cells. We showed that the inhibition of GSK3β is mediated through PKA activation triggered by IL-24. IL-24 decreases the phosphorylation of glycogen synthase, substantially activating glycogen synthase and decreasing intracellular glucose levels. Notably, the expression of a constitutively active form of GSK3β abolishes the effect of IL-24. These results demonstrate a previously unrecognized role of IL-24 in apoptosis mediated through GSK3β regulation and its possible implications for metabolic stress, mitochondria dysfunction, and apoptosis. Future studies should precisely delineate the most effective combinations of IL-24 as a GSK3β inhibitor with cytotoxic agents for prostate and other cancers. GSK3β inhibition disrupts average glucose utilization in cancer cells, potentially creating metabolic stress that could be exploited therapeutically.