Abstract: Mandarin fish ranavirus (MRV), a highly pathogenic iridovirus, causes severe tissue damage and high mortality in largemouth bass (Micropterus salmoides), resulting in substantial economic losses. MRV infection induces substantial elevation of host reactive oxygen species (ROS), which mediates multi-tissue inflammatory injury and triggers cellular apoptosis, thereby facilitating viral immune evasion. The antioxidant system is essential for ROS scavenging and preservation of cellular homeostasis. Of the antioxidant enzymes, superoxide dismutase (SOD)-driven antioxidant defense is especially pivotal in mitigating intracellular oxidative stress resulting from ROS accumulation. Eriodictyol (ER), a naturally occurring flavonoid, exhibits potent antioxidant and anti-inflammatory properties. It activates the nuclear factor erythroid 2-related factor 2 (nrf2) signaling cascade, modulates the transcription of downstream antioxidant-responsive genes, augments antioxidant enzyme activities, and ameliorates systemic oxidative stress. This study explored whether ER exerts anti-MRV activity by activating the nrf2-keap1/sod antioxidant pathway. The role of sod against MRV infection was examined via overexpression and siRNA-mediated knockdown in mandarin fish brain SCB3 cells. The antioxidant and antiviral effects of ER were assessed in SCB3 cells and M. salmoides using CC₅₀/EC₅₀ measurement, RT-qPCR, Western blot, flow cytometry, H.E staining, immunohistochemistry, and immunofluorescence assay. sod overexpression markedly inhibited MRV proliferation in SCB3 cells, whereas sod knockdown promoted viral replication. In SCB3 cells, ER upregulated the expression of nrf2 and its downstream targets sod and glutathione peroxidase 4 (gpx4a), with CC₅₀ and EC₅₀ values of 43.76 μmol/L and 8.522 μmol/L, respectively. In vivo, intragastric administration of ER upregulated hepatic nrf2, sod, and gpx4a expression, enhanced SOD and total antioxidant capacity (T-AOC) activities, reduced ROS levels from 49.7% to 12.4%, and attenuated virus-induced liver injury in MRV-infected M. salmoides. ER treatment achieved a 99.99% inhibition rate against MRV and increased host survival from 27% to 63%. Collectively, these results indicate that ER activates the antioxidant system via the nrf2-keap1/sod pathway to scavenge ROS and alleviate tissue damage, which underlies its anti-MRV activity. This study provides a theoretical basis for the application of ER in the prevention and control of MRV infection.