Inhibition of protein phosphatase 2A (PP2A) activity has been identified as a prerequisite for the transformation of human cells. However, the molecular mechanisms by which PP2A activity is inhibited in human cancers are currently unclear. In this study, we describe a cellular inhibitor of PP2A with oncogenic activity. The protein, designated Cancerous Inhibitor of PP2A (CIP2A), interacts directly with the oncogenic transcription factor c-Myc, inhibits PP2A activity toward c-Myc serine 62 (S62), and thereby prevents c-Myc proteolytic degradation. In addition to its function in c-Myc stabilization, CIP2A promotes anchorage-independent cell growth and in vivo tumor formation. The oncogenic activity of CIP2A is demonstrated by transformation of human cells by overexpression of CIP2A. Importantly, CIP2A is overexpressed in two common human malignancies, head and neck squamous cell carcinoma (HNSCC) and colon cancer. Thus, our data show that CIP2A is a human oncoprotein that inhibits PP2A and stabilizes c-Myc in human malignancies.