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The tumor suppressor protein p53 is a critical stress response transcription factor that induces the expression of genes leading to cell cycle arrest, apoptosis, and tumor suppression. We found that mammalian inositol polyphosphate multikinase (IPMK) stimulated p53-mediated transcription by binding to p53 and enhancing its acetylation by the acetyltransferase p300 independently of its inositol phosphate and lipid kinase activities. Genetic or RNA interference (RNAi)–mediated knockdown of IPMK resulted in decreased activation of p53, decreased recruitment of p53 and p300 to target gene promoters, abrogated transcription of p53 target genes, and enhanced cell viability. Additionally, blocking the IPMK-p53 interaction decreased the extent of p53-mediated transcription. These results suggest that IPMK acts as a transcriptional coactivator for p53 and that it is an integral part of the p53 transcriptional complex facilitating cell death.