Caffeine activates tumor suppressor PTEN in sarcoma cells. The tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a negative regulator of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway. Akt activation exerts a strong anti-apoptotic effect and inhibits key pro-apoptotic proteins. We investigated the effect of caffeine in the prevention of tumor cell proliferation and induction of cell death. We found that caffeine induced increased intracellular cAMP levels, PTEN activation and Akt inactivation, which together prevented proliferation of human osteosarcoma cells (MG63) and fibrosarcoma cells (HT1080). PTEN knockdown by siRNA reduced the effects of caffeine on Akt inactivation in osteosarcoma cells. These results indicate that the tumor suppressor PTEN signaling pathway contributes to the growth-inhibitory effect of caffeine on sarcoma cells. Our data suggest that caffeine and other drugs that act on this pathway could have promising therapeutic effects in the treatment of sarcoma patients.