Liver is considered the critical tissue for metabolic processing of dietary nutrients including lipids. High-fat diets (HFD) can induce hepatic lipotoxic injury, but the mechanism is unclear. This study aimed to elucidate the underlying role of endoplasmic reticulum stress (ERS) in mediating HFD-induced hepatic lipotoxic injury in black seabream (Acanthopagrus schlegelii) via in vitro and in vivo experiments. For the in vitro experiment, isolated hepatocytes from black seabream were incubated for 24 h in medium (Control), or medium supplemented with 200 ÎŒM oleic acid (OA) alone or in combination with 20 ÎŒM 4-phenyl butyric acid (OA + 4-PBA). In the in vivo experiment, black seabream juveniles (3.30 ± 0.10 g) were fed diets containing lipid at 12.4% (Control) or 18.6% (HFD) for 8 weeks, after which the HFD-fed fish were divided into three groups and, on alternate days over a period of 8 days, subjected to intraperitoneal injections of 50 mg/kg 4-PBA in hydroxypropyl-ÎČ-cyclodextrin (HBC) (treatment 4-PBA) or carrier alone (treatment HFD + HBC) versus no injection (HFD). The contents of triglyceride and non-esterified fatty acid, as well as expression levels of genes and proteins involved in lipid metabolism, ERS, apoptosis and inflammation were determined in liver and isolated hepatocytes, and oil-red O staining, ultrastructural observation, flow cytometry assay of apoptosis rate, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay (TUNEL) staining were performed on hepatocytes. The in vitro experiment showed that 4-PBA treatment significantly decreased lipid accumulation, and the OA-induced expression levels of genes/proteins linked to ERS, lipogenesis, inflammation and apoptosis in hepatocytes. Transmission Electron Microscope (TEM) imaging showed that 4-PBA treatment mitigated the ER swelling induced by OA. These results were confirmed in the in vivo experiment, where 4-PBA injection reduced lipid deposition and expression levels of genes/proteins related to ERS, lipogenesis, inflammation and apoptosis in liver compared to the HFD treatment. In conclusion, these results confirmed that ERS plays a contributing regulatory role in HFD- and OA-induced lipotoxic damage including lipid accumulation, apoptosis and inflammation in liver and isolated hepatocytes of black seabream, which supply a new perspective in the cross-talk of ERS and hepatic lipotoxic damage in hepatocytes/liver of marine fish and provide theoretical guidance for the prevention and treatment of hepatic lipotoxic damage.