It has been widely accepted that 5-methylcytosine is the only form of DNA methylation in mammalian genomes. Here we identify N⁶-methyladenine as another form of DNA modification in mouse embryonic stem cells. Alkbh1 encodes a demethylase for N⁶-methyladenine. An increase of N⁶-methyladenine levels in Alkbh1-deficient cells leads to transcriptional silencing. N⁶-methyladenine deposition is inversely correlated with the evolutionary age of LINE-1 transposons; its deposition is strongly enriched at young (<1.5 million years old) but not old (>6 million years old) L1 elements. The deposition of N⁶-methyladenine correlates with epigenetic silencing of such LINE-1 transposons, together with their neighbouring enhancers and genes, thereby resisting the gene activation signals during embryonic stem cell differentiation. As young full-length LINE-1 transposons are strongly enriched on the X chromosome, genes located on the X chromosome are also silenced. Thus, N⁶-methyladenine developed a new role in epigenetic silencing in mammalian evolution distinct from its role in gene activation in other organisms. Our results demonstrate that N⁶-methyladenine constitutes a crucial component of the epigenetic regulation repertoire in mammalian genomes.