Alpha-fetoprotein (AFP) elevation is a well-known biomarker in various diseases, particularly in the diagnosis of hepatocellular carcinoma (HCC). Intracellular AFP has been previously implicated in promoting tumorigenesis. In this study, we discovered that AFP enhances the stability of oncoproteins c-MYC and c-MET, thereby facilitating the progression of liver and gastric tumors. Our findings suggest that AFP acts by stabilizing these oncoproteins, which are clients of heat shock protein 90 (HSP90), and prevents their degradation through ubiquitination. Intriguingly, we identified AFP as a novel co-chaperone of HSP90, demonstrating its ability to regulate the stabilization of HSP90 client proteins. Furthermore, our results indicate that inhibiting AFP or HSP90 enhances the cytotoxicity of chemotherapeutic agents in AFP-producing HCC and gastric cancer cells. These findings have significant implications for the development of therapeutic strategies targeting AFP-producing tumors, as the AFP-HSP90-mediated activation of c-MYC and c-MET provides new insights into potential treatment approaches. In summary, this study sheds light on the role of AFP in promoting tumor progression by stabilizing oncoproteins through its interaction with HSP90. The identification of this mechanism opens up new avenues for therapeutic interventions in AFP-producing tumors.