The promyelocytic leukemia (PML) protein is a potent tumor suppressor and proapoptotic factor, and is functionally regulated by posttranslational modifications such as phosphorylation, sumoylation, and ubiquitination. Histone deacetylase (HDAC) inhibitors are a promising class of targeted anticancer agents and induce apoptosis in cancer cells by largely unknown mechanisms. We report here a novel posttranscriptional modification, acetylation, of PML. PML exists as an acetylated protein in HeLa cells and its acetylation is enhanced by coexpression of p300 or treatment with a histone deacetylase (HDAC) inhibitor, trichostatin A (TSA). Increased PML acetylation is associated with increased sumoylation of PML in vitro and in vivo. PML is involved in TSA-induced apoptosis and PML with an acetylation-defective mutation shows an inability to mediate apoptosis, suggesting the importance of PML acetylation. Our work provides new insights into PML regulation by posttranslational modification, and new information about the therapeutic mechanism of HDAC inhibitors.