Significance Enhancer of Zeste Homolog 2 (EZH2) is the catalytic subunit of Polycomb Repressive Complex 2, mediating trimethylation of histone H3K27, a hallmark of gene silencing. Paradoxically, accumulating evidence indicates that EZH2 also dictates a unique gene activation program to promote cancer development. The molecular mechanism underlying the functional conversion of EZH2 from a gene repressor to an activator is unclear. In the current study, we report an unexpected finding that EZH2 contains a hidden, partially disordered transactivation domain (TAD), which directly binds to the transcriptional coactivator p300 and activates gene expression in cancer cells. Structural analysis suggests a “lock-and-release” mechanism of the functional control of the EZH2 TAD by cancer-specific EZH2 phosphorylation.