<p>N⁶-Methyladenosine (m⁶A) is the most abundant modification in mRNAs and regulates RNA splicing, stabilization, translation and export. FTO was the first discovered RNA m⁶A demethylase, and it is frequently dysregulated in cancers and plays important roles in tumorigenesis, cancer cell stemness, immune evasion, metabolic programs and drug resistance. Here, we report that FTO is acetylated at lysine 88 (K88) by the acetyltransferase GCN5 in vivo and in vitro. K88 acetylation significantly increases the RNA m⁶A demethylase activity of FTO. Acetylation of FTO at K88 has no effect on its stability, localization or dimerization but significantly increases its binding to m⁶A-modified RNA, thereby facilitating the removal of m⁶A from RNA. K88 acetylation is markedly increased in cancers, and the elimination of K88 acetylation in FTO inhibits the ability of FTO to stimulate tumorigenesis. K88 acetylation facilitates a reduction in the m⁶A level of FTO target mRNAs MYC, PDGFC, SOX10, CXCR4, NRF2, PLD1 and CDC42 and subsequently alters the stability or translation of these target mRNAs, thereby promoting tumorigenesis. Hence, K88 acetylation is critical for the RNA m⁶A demethylase activity and tumor-promoting functions of FTO.</p>