BACKGROUND & AIMS: Recent findings reveal the importance of tryptophan-initiated de novo nicotinamide adenine dinucleotide (NAD) synthesis in the liver, a process previously considered secondary to biosynthesis from nicotinamide. The enzyme α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD), primarily expressed in the liver and kidney, acts as a modulator of de novo NAD synthesis. Boosting NAD levels has previously demonstrated remarkable metabolic benefits in mouse models. In this study, we aimed to investigate the therapeutic implications of ACMSD inhibition in the treatment of metabolic dysfunction-associated steatotic liver disease/steatohepatitis (MASLD/MASH). METHODS: In vitro experiments were conducted in primary rodent hepatocytes, Huh7 human liver carcinoma cells and induced pluripotent stem cell-derived human liver organoids (HLOs). C57BL/6J male mice were fed a western-style diet and housed at thermoneutrality to recapitulate key aspects of MASLD/MASH. Pharmacological ACMSD inhibition was given therapeutically, following disease onset. HLO models of steatohepatitis were used to assess the DNA damage responses to ACMSD inhibition in human contexts. RESULTS: Inhibiting ACMSD with a novel specific pharmacological inhibitor promotes de novo NAD synthesis and reduces DNA damage ex vivo, in vivo, and in HLO models. In mouse models of MASLD/MASH, de novo NAD biosynthesis is suppressed, and transcriptomic DNA damage signatures correlate with disease severity; in humans, Mendelian randomization-based genetic analysis suggests a notable impact of genomic stress on liver disease susceptibility. Therapeutic inhibition of ACMSD in mice increases liver NAD and reverses MASLD/MASH, mitigating fibrosis, inflammation, and DNA damage, as observed in HLO models of steatohepatitis. CONCLUSIONS: Our findings highlight the benefits of ACMSD inhibition in enhancing hepatic NAD levels and enabling genomic protection, underscoring its therapeutic potential in MASLD/MASH. IMPACT AND IMPLICATIONS: Enhancing NAD levels has been shown to induce remarkable health benefits in mouse models of metabolic dysfunction-associated steatotic liver disease/steatohepatitis (MASLD/MASH), yet liver-specific NAD boosting strategies remain underexplored. Here, we present a novel pharmacological approach to enhance de novo synthesis of NAD in the liver by inhibiting α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD), an enzyme highly expressed in the liver. Inhibiting ACMSD increases NAD levels, enhances mitochondrial respiration, and maintains genomic stability in hepatocytes ex vivo and in vivo. These molecular benefits prevent disease progression in both mouse and human liver organoid models of steatohepatitis. Our preclinical study identifies ACMSD as a promising target for MASLD/MASH management and lays the groundwork for developing ACMSD inhibitors as a clinical treatment.