Abstract Body: Atherosclerotic cardiovascular disease (ASCVD) is the major cause of death in patients with metabolic dysfunction-associated steatohepatitis (MASH), independent of traditional risk factors. This underscores an urgent need for novel therapeutic targets that address both MASH and ASCVD concurrently. While dysregulated lipid and amino acid metabolism is common in both MASH and ASCVD, the interaction between these two diseases remain largely unexplored. N-acyl amino acids (NAAs), endogenous signaling molecules comprising amino acids linked to long-chain fatty acid acyl groups, lie at the intersection of lipid and amino acid metabolism. Our metabolomic analysis revealed a marked reduction in hepatic NAAs in MASH, with levels of specific NAAs, particularly N-oleoyl leucine (C18:1-Leu), inversely correlating correlated with MASH severity and inflammatory indices. Unbiased transcriptomics, validated by qPCR and western blotting, revealed suppression of NAA biosynthetic enzymes and upregulation of NAA degradative enzymes in human and mouse livers with MASH, as well as in lipid-loaded primary mouse hepatocytes and hepatic cell lines. Stable isotope tracing studies in mice with and without MASH confirmed reduced biosynthesis of C18:1-Leu. Importantly, hepatocyte specific overexpression of peptidase M20 domain containing 1 (PM20D1), which drives C18:1-Leu biosynthesis, significantly attenuated MASH progression and ameliorated established MASH. Notably, treatment with exogenous C18:1-Leu attenuated MASH in both male and female mice. Mechanistically, C18:1-Leu activated peroxisome proliferator-activated receptor alpha (PPARα) and stimulated fatty acid β-oxidation (FAO), leading to marked reductions in steatohepatitis, hepatic macrophage accumulation, and fibrosis. Unbiased RNA sequencing demonstrated that C18:1-Leu treatment upregulated FAO and oxidative phosphorylation, while and suppresseding pro-inflammatory pathways. This was, accompanied by reduced levels of the chemokine C-C motif ligand 2 (CCL2) in both liver and plasma. These effects were partially dependent on PPARα, as evidenced by studies in hepatocyte specific PPARα-deficient (Ppara^HKO) mice. In Apoe^-/- mice, C18:1-Leu lowered hepatic steatosis and CCL2, while reducing lesional macrophages and atherosclerosis. In summary, our findings reveal a novel metabolic pathway that bridges MASH and ASCVD, highlighting NAAs as promising dual-targeted therapeutic agents for these interlinked diseases.