N⁶-methyladenosine (m⁶A) is the most prevalent and reversible internal modification in mammalian messenger and noncoding RNAs. We report here that human methyltransferase-like 14 (METTL14) catalyzes m⁶A RNA methylation. Together with METTL3, the only previously known m⁶A methyltransferase, these two proteins form a stable heterodimer core complex of METTL3–METTL14 that functions in cellular m⁶A deposition on mammalian nuclear RNAs. WTAP, a mammalian splicing factor, can interact with this complex and affect this methylation.