Inhibition of endoplasmic reticulum stress (ERS) can effectively improve the progression of metabolic dysfunction-associated steatotic liver disease (MASLD). In this study, we attempted to further explore the mechanism of Fam172a in high fat diet (HFD) induced-MASLD.The correlation between FAM172A and ERS-induced MASLD was tested by WB and qRT-PCR. We utilized wild type (WT) and Fam172a gene knockout (Fam172a-/-) mice to build two models: MASLD model induced by HFD and ERS model induced by tunicamycin (Tm). We evaluated the degree of liver inflammation, steatosis, and lipid accumulation, and key molecules of ERS and apoptosis (Bax/Bcl-2). Based on RNA-seq analysis, we verified downstream target molecules of Fam172a. Co-immunoprecipitation was used to explore the binding protein of Fam172a and its core functional fragment.We have found that Fam172a-/- mice with MASLD shows significantly obesity, dysfunction of glucolipid metabolism, severe hepatic inflammation and steatosis, and ERS pathway activation. Similar results are found in the ERS mouse model. We also reveal that Fam172a-/- may significantly up-regulate fibroblast growth factor 21 (Fgf21) expression, probably through activating eukaryotic translation initiation factor 2 alpha (eIF2α). FGF21 can partly counteract Fam172a deletion-induced ERS and hepatic steatosis. Fam172a 1-102 amino acids is the core region to interact with eIF2α.Fam172a gene deletion can promote HFD-induced MASLD via the eIF2α-ATF4 pathway. The expression of Fgf21, up-regulated by the eIF2α-ATF4 pathway, partially inhibit the effects of Fam172a gene deletion via negative feedback loop. Thus, FAM172A may serve as a potential target of MASLD.Copyright © 2025. Published by Elsevier Inc.