Significance A high frequency of missense mutations was recently discovered at histone H3.3 glycine 34 in 92% of giant cell tumors of the bone and 17% of high-grade astrocytomas. The molecular mechanism by which G34 mutations drive these tumors remains unclear. Here, we demonstrate that the G34-mutated “oncohistones” misregulate the negative cross-talk between two histone methyltransferase enzymes, Polycomb Repressive Complex 2 (PRC2) and SETD2. G34 mutations uniquely promote PRC2 activity by blocking SETD2-mediated H3K36 methylation at active enhancers and drive a gene expression program that enhances tumor growth. We propose that G34 oncohistones exploit the regulatory mechanisms that fine tune PRC2 activity in human malignancies.