Tumor-infiltrating myeloid cells (TIMs) are crucial cell populations involved in tumor immune escape, and their functions are regulated by multiple epigenetic mechanisms. The precise regulation mode of RNA N6-methyladenosine (m6A) modification in controlling TIM function is still poorly understood. Our study revealed that the increased expression of methyltransferase-like 3 (METTL3) in TIMs was correlated with the poor prognosis of colon cancer patients, and myeloid deficiency of METTL3 attenuated tumor growth in mice. METTL3 mediated m6A modification on Jak1 mRNA in TIMs, the m6A-YTHDF1 axis enhanced JAK1 protein translation efficiency and subsequent phosphorylation of STAT3. Lactate accumulated in tumor microenvironment potently induced METTL3 upregulation in TIMs via H3K18 lactylation. Interestingly, we identified two lactylation modification sites in the zinc-finger domain of METTL3, which was essential for METTL3 to capture target RNA. Our results emphasize the importance of lactylation-driven METTL3-mediated RNA m6A modification for promoting the immunosuppressive capacity of TIMs. Display omitted •H3K18 lactylation increases Mettl3 expression in tumor-infiltrating myeloid cells•METTL3-mediated m6A modification on Jak1 mRNA promotes its protein translation•METTL3/m6A/JAK1/STAT3 axis strengthens immunosuppressive functions of myeloid cells•Lactylation on zinc-finger domain of METTL3 enhances its capture of m6A-modified RNA Xiong et al. discovered that lactate in tumor microenvironment is a key factor for inducing the expression and function of RNA methyltransferase METTL3. METTL3-mediated m6A modification potently enhances the immunosuppressive functions of tumor-infiltrating myeloid cells to promote tumor immune escape.