It is well-established that Largemouth bass ranavirus (LMBRaV) induces viral ulcerative syndrome, a highly transmissible and lethal viral disease afflicting largemouth bass aquaculture. Given the urgent need for available prevention and control methods, this study evaluated the antiviral activities of fifteen naturally occurring compounds derived from herbal medicines against LMBRaV using an in vitro cell model. Andrographolide (Andro) was one of the substances that was evaluated and showed highly effective anti-LMBRaV activity of a 99.2 % maximal inhibition rate of at 20 mg/L dosage. Mechanistic investigation revealed Andro prevented viral entry and directly inactivated the virus. Ultrastructural observations and fluorescence staining further supported Andro's protective effects on EPC cells from LMBRaV invasion. In vivo experiments demonstrated Andro's efficacy in reducing mortality rates in LMBRaV-infected largemouth bass by 61.54 % and significantly lowering the viral load. Additionally, Andro upregulated antiviral genes such as ifn-γ, irf-3, and mx, indicating its immunomodulatory properties. Histopathological analysis revealed that Andro alleviated tissue damage of liver, spleen, and kidney in the infected fish. Andro also changed the microbiotic composition of gut by increasing the amounts of good bacteria and reducing amounts of bad bacteria. These findings collectively highlight Andro's substantial antiviral and immunomodulatory potential, suggesting its promising application as a natural compound for combating LMBRaV infections in largemouth bass aquaculture and potentially other aquatic animal diseases.