The giant grouper (Epinephelus lanceolatus) is a euryhaline aquaculture species capable of tolerating salinities ranging from freshwater (0���) to hypersaline environments (50���). Despite its remarkable salinity tolerance, the cellular mechanisms underlying this physiological plasticity, and their interaction with antiviral defense, remain incompletely understood. This study examined gill ionocyte morphology, distribution, and Na+/K+-ATPase (NKA) responses in juvenile giant groupers acclimated to five salinity conditions (0���, 5���, 10���, 35���, and 50���), and further evaluated their responses following infection with nervous necrosis virus (NNV). Following NNV challenge, antiviral responses displayed salinity-dependent modulation, with fish maintained near seawater conditions (35���) exhibiting more robust antiviral capacity, whereas extreme salinities were associated with altered host responses and susceptibility to viral infection. Despite substantial gill remodeling, muscle water content and plasma osmolality remained stable across treatments, indicating effective systemic osmoregulatory compensation. Collectively, these findings reveal coordinated structural and molecular plasticity of the grouper gill in response to salinity variation and demonstrate that environmental salinity critically shapes host–virus interactions. This study highlights the role of NKA-mediated ionocyte regulation as a key mechanism supporting extreme euryhalinity, while also uncovering a functional trade-off between osmoregulation and antiviral defense. These insights provide a mechanistic basis for optimizing salinity conditions to improve disease resistance and inform salinity-flexible aquaculture strategies.