A METTL3-METTL14 complex mediates mammalian nuclear RNA N⁶-adenosine methylation. The study identifies METTL14 as a new methyltransferase that forms a stable heterodimer with METTL3, which is the only previously known methyltransferase for N⁶-adenosine (m⁶A) RNA methylation. Together, these proteins form a stable complex that functions in m⁶A deposition on mammalian nuclear RNAs. WTAP, a mammalian splicing factor, can interact with this complex and affect this methylation. The m⁶A modification is post-transcriptionally installed by a multi-component N⁶-adenosine methyltransferase (MT) complex. METTL3 and METTL14 are highly conserved in mammals and form a stable complex. The study shows that METTL3 and METTL14 preferentially methylate RNA substrates containing the previously revealed consensus sequence. The METTL3-14 complex and recently discovered m⁶A RNA demethylases dynamically regulate m⁶A in mRNA and other nuclear RNA through opposing enzymatic functions. The study also reveals that the METTL3-14 complex is enriched in nuclear extracts and co-localizes with splicing factors in nuclear speckles. PAR-CLIP analysis identifies binding sites for METTL3, METTL14, and WTAP, which are consistent with the previously identified consensus sequence for m⁶A. The study further shows that the METTL3-14 complex is involved in m⁶A deposition on nuclear RNA and that WTAP interacts with this complex to affect m⁶A deposition. The results indicate that m⁶A methylation has regulatory functions in gene expression, consistent with an m⁶A-dependent mRNA degradation process. The study provides a basis for future mechanistic investigations of biological functions associated with m⁶A deposition.A METTL3-METTL14 complex mediates mammalian nuclear RNA N⁶-adenosine methylation. The study identifies METTL14 as a new methyltransferase that forms a stable heterodimer with METTL3, which is the only previously known methyltransferase for N⁶-adenosine (m⁶A) RNA methylation. Together, these proteins form a stable complex that functions in m⁶A deposition on mammalian nuclear RNAs. WTAP, a mammalian splicing factor, can interact with this complex and affect this methylation. The m⁶A modification is post-transcriptionally installed by a multi-component N⁶-adenosine methyltransferase (MT) complex. METTL3 and METTL14 are highly conserved in mammals and form a stable complex. The study shows that METTL3 and METTL14 preferentially methylate RNA substrates containing the previously revealed consensus sequence. The METTL3-14 complex and recently discovered m⁶A RNA demethylases dynamically regulate m⁶A in mRNA and other nuclear RNA through opposing enzymatic functions. The study also reveals that the METTL3-14 complex is enriched in nuclear extracts and co-localizes with splicing factors in nuclear speckles. PAR-CLIP analysis identifies binding sites for METTL3, METTL14, and WTAP, which are consistent with the previously identified consensus sequence for m⁶A. The study further shows that the METTL3-14 complex is involved in m⁶A deposition on nuclear RNA and that WTAP interacts with this complex to affect m⁶A deposition. The results indicate that m⁶A methylation has regulatory functions in gene expression, consistent with an m⁶A-dependent mRNA degradation process. The study provides a basis for future mechanistic investigations of biological functions associated with m⁶A deposition.