Abstract: Silymarin is a class of natural flavonoid compounds extracted from the seeds of Silybum marianum (Asteraceae), which possess diverse biological activities including antioxidant, anti-inflammatory, hepatoprotective, and lipid metabolism-regulating properties. In mammalian studies, silymarin has been proven to effectively alleviate hepatic lipid deposition and oxidative stress injury induced by high-fat diets. However, relevant research regarding its application effects and underlying mechanisms in aquatic animals remains scarce. In this experiment, largemouth bass (Micropterus salmoides) were used as the research object to investigate the effects of adding silymarin to high-fat diets on the growth performance, hepatic lipid metabolism, and antioxidant capacity of the fish. A total of 360 largemouth bass with an initial body weight of (3.20 ± 0.10) g were selected and randomly divided into 4 groups: the normal fat diet group (NFD), the high-fat diet group (HFD), the high-fat diet + 0.02% silymarin group (HSM1), and the high-fat diet + 0.04% silymarin group (HSM2). Each group had 3 replicates, with 30 fish per replicate, and the culture trial lasted for 8 weeks. The results demonstrated that dietary supplementation with silymarin could significantly mitigate the decline in growth performance of largemouth bass induced by high-fat diet. Specifically, it reversed the significant decreases in final body weight, weight gain rate, feed intake, and specific growth rate (P < 0.05), as well as the marked increases in feed conversion ratio, hepatosomatic index, and viscerosomatic index (P < 0.05) caused by the high-fat diet. Meanwhile, silymarin ameliorated lipid metabolism disorders, as reflected by reduced triglyceride levels, up-regulated expression of lipid catabolism genes (pparα and cpt1), and down-regulated expression of lipid synthesis genes (pparγ and fas). It also alleviated hepatic oxidative stress, characterized by a decrease in malondialdehyde content and increased activities of antioxidant enzymes such as superoxide dismutase and catalase. Additionally, it significantly attenuated the marked elevation of 15-lipoxygenase and its metabolite 15-HETE in the liver (P < 0.05). In conclusion, high-fat diets can significantly inhibit the growth performance of largemouth bass, disrupt the homeostasis of hepatic lipid metabolism, and induce obvious oxidative stress in the liver. In contrast, dietary supplementation with appropriate dosages of silymarin can effectively alleviate these adverse effects caused by high-fat diets by regulating the expression of key genes involved in lipid metabolism and enhancing the antioxidant capacity of the organism. This study not only clarifies the potential application value of silymarin in high-fat diets for largemouth bass, but also provides reliable empirical data to support the optimization of high-fat diets and the development of functional feed additives for aquatic animals, thereby laying a theoretical foundation for its practical application in the aquafeed industry.