Malignant melanoma is the most invasive and deadly form of skin cancer with no effective therapy to treat advanced disease, leading to poor survival rates. Akt3 signaling plays an important role in deregulating apoptosis in ∼70% of melanomas. Thus, targeting Akt3 signaling in melanoma patients has significant therapeutic potential for inhibiting melanomas, but no Akt3-specific chemotherapeutic agent exists. Unfortunately, nonspecific Akt inhibitors can cause systemic toxicity or increase metastasis. Identifying and targeting the Akt3 substrate that deregulates apoptosis might circumvent these complications but would require demonstration of its functional importance in disrupting normal apoptosis. In this study, PRAS40 was identified as an Akt3 substrate that deregulated apoptosis to promote melanoma tumorigenesis. Levels of phosphorylated PRAS40 (pPRAS40) increased during melanoma tumor progression paralleling increasing Akt3 activity. Majority of melanomas from patients with elevated Akt activity also had correspondingly higher levels of pPRAS40. Targeting PRAS40 or upstream Akt3 similarly reduced anchorage-independent growth in culture and inhibited tumor development in mice. Mechanistically, decreased pPRAS40 increased tumor cell apoptosis as well as sensitivity of melanoma cells to apoptosis-inducing agents, thereby decreasing chemoresistance. Collectively, these studies provide a solid mechanistic basis for targeting PRAS40 to inhibit the Akt3 signaling cascade and thereby retard melanoma development. [Cancer Res 2007;67(8):3626–36]