Chemical modification of RNAs is important for post-transcriptional gene regulation. The METTL3-METTL14 complex generates most N 6 -methyladenosine (m 6 A) modifications in mRNAs, and dysregulated methyltransferase expression has been linked to numerous cancers. Here we show that changes in m 6 A modification location can impact oncogenesis. A gain-of-function missense mutation found in cancer patients, METTL14 R298P , promotes malignant cell growth in culture and in transgenic mice. The mutant methyltransferase preferentially modifies noncanonical sites containing a GGAU motif and transforms gene expression without increasing global m 6 A levels in mRNAs. The altered substrate specificity is intrinsic to METTL3-METTL14, helping us to propose a structural model for how the METTL3-METTL14 complex selects the cognate RNA sequences for modification. Together, our work highlights that sequence-specific m 6 A deposition is important for proper function of the modification and that noncanonical methylation events can impact aberrant gene expression and oncogenesis.