<p>Methionine metabolism generates the substrate S-adenosylmethionine (SAM), which regulates epigenetic modifications crucial for various cellular processes, particularly tumorigenesis. However, whether methionine metabolism involves epigenetic mechanisms independent of SAM and what roles such mechanisms play in tumorigenesis remain unclear. We show here that the adenosylhomocysteinase (AHCY)–adenosine complex increases mRNA m⁶A levels in a non-global manner, promoting fatty acid synthesis and tumorigenesis. Adenosine increases mRNA m⁶A levels by binding to the methionine metabolism enzyme AHCY to form a complex, rather than depending on adenosine receptors. The AHCY–adenosine complex facilitates AHCY dimerization, with adenosine being crucial for dimer stability. AHCY dimers hinder the binding of fat mass and obesity-associated protein (FTO) at the Q86 site to RNA containing the VWDRACH motif, increasing m⁶A levels and upregulating lipogenesis genes, especially ACACA and SCD1, thus leading to reprogramming of lipid metabolism. Conversely, AHCY mutants that have lost dimerization or FTO-binding ability but retain hydrolase activity suppress lipogenesis and tumor growth without significantly affecting methionine catabolism mediated by AHCY. Loss of AHCY in mice and disruption of AHCY dimerization in tumor cells and patient-derived xenograft models restricted tumor growth. Our findings demonstrate a key SAM-independent link between methionine metabolism and mRNA m⁶A modification that affects demethylase substrate specificity. This novel link between the methionine cycle and lipid metabolism suggests new strategies for anticancer therapy.</p>