Tumor suppressor p53 is often activated in response to DNA damage or other forms of stress, leading to either cell cycle arrest or apoptosis. Stress-induced kinases phosphorylate p53 thereby enhancing its stability, leading to an increase in transactivation of its target genes. Several different protein kinases phosphorylate p53 on multiple amino acid residues. Here, we report for the first time that Cyclin dependent kinase 9, whose well-known substrate is RNA polymerase II, can also phosphorylate p53. Specifically, Ser33 on the N-terminus and, Ser315 and Ser392 on the C-terminus of p53 were found to be phosphorylated. The precise biological role of this phosphorylation remains to be elucidated.