Abstract: Largemouth bass ranavirus (LMBV) is a major pathogen in largemouth bass (Micropterus salmoides) aquaculture, frequently causing high mortality and substantial economic losses. Non-specific immune responses are crucial for host antiviral defence. Although interferons are known to mediate antiviral immunity in teleosts, the function and underlying mechanisms of type II interferon IFN-γ (interferon-γ) in largemouth bass resistance to LMBV infection remain insufficiently elucidated. To address this, recombinant rMsIFN-γ was expressed and purified using a pCold-TF/Rosetta (DE3) prokaryotic system, and its bioactivity and anti-LMBV effects were assessed both in vitro using the Micropterus salmoides spinal cord cell line (MSSC) and in vivo. The results showed that rMsIFN-γ had an apparent molecular mass of approximately 21 kDa. In vitro, rMsIFN-γ markedly upregulated the expression of Mx, ISG15, SOCS1, and IRF9 in MSSC cells, and exhibited measurable anti-LMBV activity under both prophylactic and therapeutic treatment regimens. In vivo, rMsIFN-γ induced the expression of Mx, ISG15, SOCS1, and IRF9, as well as the pro-inflammatory cytokines IL-1β and TNF-α and the chemokine IL-8, in the spleen and head kidney. In therapeutic experiments, early rMsIFN-γ administration after infection resulted in lower Mx and ISG15 expression than that in virus-infected controls, while IL-1β and IL-8 were significantly elevated; concomitantly, expression of the LMBV immediate-early protein ICP-46 was reduced. Thereafter, viral loads in tissues decreased and mortality progression in infected fish was delayed. Collectively, rMsIFN-γ confers demonstrable anti-LMBV effects both in vitro and in vivo, likely associated with the canonical interferon-stimulated genes (ISGs) pathway in conjunction with coordinated inflammatory and chemotactic responses in vivo. These findings highlight the potential utility of IFN-γ for LMBV control and provide a novel perspective for immunological prevention and intervention strategies in largemouth bass.