Aberrant RNA splicing is tightly linked to diseases, including metabolic dysfunction-associated steatotic liver disease (MASLD). In this study, we revealed that minor intron splicing, a unique and conserved RNA processing event, is largely disrupted upon the progression of metabolic dysfunction-associated steatohepatitis (MASH) in mice and humans. We demonstrated that deficiency of minor intron splicing in the liver induced MASH transition upon obesity-induced insulin resistance and LXR activation. Mechanistically, inactivation of minor intron splicing led to minor intron retention of Insig1 and Insig2, resulting in premature termination of translation, which drove proteolytic activation of SREBP1c. This mechanism was conserved in patients with MASH. Notably, disrupted minor intron splicing activated glutamine reductive metabolism for de novo lipogenesis through induction of Idh1, which caused accumulation of ammonia in the liver, thereby initiating hepatic fibrosis upon LXR activation. Ammonia clearance or IDH1 inhibition blocked hepatic fibrogenesis and mitigated MASH progression. More importantly, overexpression of Zrsr1 restored minor intron retention and ameliorated the development of MASH, indicating that dysfunctional minor intron splicing is an emerging pathogenic mechanism that drives MASH progression. Additionally, our results suggest that reductive carboxylation flux triggered by minor intron retention in hepatocytes serves as a crucial checkpoint and potential target for MASH therapy.