Abstract The Warburg effect released lactate promotes cancer progression, but the mechanisms remain unclear. Here, we found lactate activated MAPK pathway through ERK-lactylation to promote cancer progression. Moreover, we identified the GCN5 as the lactyl-transferase for ERK lactylation. Interestingly, activated ERK phosphorylated GCN5 and promoted GCN5 lactyl-transferase activity for ERK, which formed the positive feedback loop to facilitate lactate-mediated cancer progression. Mechanistically, ERK-K231 lactylation decreased the dissociation energy between ERK and MEK, due to the reduced electrostatic interaction between ERK-K231 and MEK-D217. This facilitated the dissociation of ERK from MEK kinases, which in turn induced ERK dimerization and activation. Hence, we developed a cell-penetrating peptide to specifically inhibit the ERK lactylation, and demonstrated the peptide impaired the tumor growth with KRAS-mutant. Taken together, we define a molecular mechanism that lactate accelerates cancer progression through ERK-GCN5 lactylation-phosphorylation cascade and provide a strategy to target ERK lactylation, especially for RAS-MAPK-driven cancers.