Abstract: To investigate the effects of dietary rosemary extract on growth performance, liver health, and glycolipid metabolism of juvenile largemouth bass (Micropterus salmoides) under a dietary high-carbohydrate load, a diet containing 18% cassava starch was used as the control group (high-starch group, CON), and two experimental diets were supplemented with 0.05% and 0.10% rosemary extract based on the control diet, designated as R0.05 and R0.1, respectively. Juvenile largemouth bass with an initial body weight of (19.00±0.20) g were reared in cages for 8 weeks, with three replicates per treatment and 50 fish per replicate. Compared with the CON group, the R0.1 group significantly increased the weight gain rate (WGR) and intestinal trypsin activity (P<0.05). Serum biochemical indicators showed that glucose (GLU), total cholesterol (TC), and triglyceride (TG) levels were significantly decreased, and high-density lipoprotein cholesterol (HDL-C) was significantly increased in the R0.1 group; in the rosemary extract supplemented groups, low-density lipoprotein cholesterol (LDL-C) and non-esterified fatty acid (NEFA) levels were significantly decreased, along with significantly increased insulin (INS) and glucagon (GCG) levels (P<0.05). Meanwhile, rosemary extract significantly reduced hepatic glycogen (HGC) and lipid deposition, alleviated hepatocyte vacuolization and tissue damage, and significantly decreased serum GOT and GPT activities (P<0.05). Gene expression analysis showed that genes related to glycolysis (pfk) and glycogen synthesis and transport (gs, glut2) were significantly upregulated, while genes associated with gluconeogenesis (pepck, fbpase) and triglyceride synthesis (gpat4, dgat1) were significantly downregulated in the rosemary extract supplemented groups (P<0.05); furthermore, the R0.1 group exhibited a more prominent upregulatory effect on glucokinase (gk) and downregulatory effects on sterol regulatory element binding protein-1 (srebp1) and fatty acid synthesis-related genes (lxr, fasn) (P<0.05). In conclusion, under the present experimental conditions, dietary rosemary extract improved the growth and hepatic metabolic health of juvenile largemouth bass under a dietary high-carbohydrate load, with 0.10% supplementation showing relatively better effects, which can serve as a reference for the development of functional feed additives for largemouth bass.