Histone modifications and DNA methylation are epigenetic phenomena that play a critical role in many neoplastic processes, including silencing of tumor suppressor genes. One such histone modification, particularly at H3 and H4, is methylation at specific lysine (K) residues. Whereas histone methylation of H3-K9 has been linked to DNA methylation and aberrant gene silencing in cancer cells, no such studies of H3-K27 have been reported. Here, we generated a stable cell line overexpressing a dominant-negative point mutant, H3-K27R, to examine the role of that specific lysine in ovarian cancer. Expression of this construct resulted in loss of methylation at H3-K27, global reduction of DNA methylation, and increased expression of tumor suppressor genes. One of the affected genes, RASSF1, was shown to be a direct target of H3-K27 methylation-mediated silencing. By increasing DNA-platinum adduct formation, indicating increased access of the drug to target DNA sequences, removal of H3-K27 methylation resensitized drug-resistant ovarian cancer cells to the chemotherapeutic agent cisplatin. This increased platinum-DNA access was likely due to relaxation of condensed chromatin. Our results show that overexpression of mutant H3-K27 in mammalian cells represents a novel tool for studying epigenetic mechanisms and the Histone Code Hypothesis in human cancer. Such findings show the significance of H3-K27 methylation as a promising target for epigenetic-based cancer therapies.