Brackishwater fish production has increased in recent years and is poised to narrow the gap between production and demand since there are limitations in the expansion of freshwater aquaculture. Increased culture intensity and species diversification have resulted in increased disease outbreaks in brackishwater aquaculture, especially those caused by viruses. The important viral diseases affecting brackishwater fishes include infectious haematopoietic necrosis, infectious salmon anaemia, pancreas disease or sleeping disease, viral haemorrhagic septicaemia, infectious pancreatic necrosis, viral nervous necrosis and red sea bream iridoviral disease. Most of these diseases cause serious economic losses and are OIE (World Organisation for Animal Health) notifiable diseases. Of these, viral nervous necrosis and red sea bream iridoviral disease have been recorded from brackishwater fishes in India. The nature of the disease, its causative agent, transmission mechanism, pathogenesis, diagnosis and control measures, including the recent advances in vaccine development, are reviewed in this chapter. As no therapeutic drugs are available for these diseases, their control depends on adopting good management practices and strict biosecurity protocols to prevent pathogen introduction into the facility, including quarantine and screening of stocks before introduction into a hatchery or culture facility. Further, notifying disease-free zones, preventing the introduction of stocks from endemic areas, and vaccinating susceptible populations with a potent vaccine are also crucial. As vaccination by injection is not feasible in smaller fish, the development of mucosal vaccines, which can be administered orally, by immersion or through the nasal route, stimulating mucosal immunity, thereby preventing pathogen entry through these entry portals, is essential. The development and application of immunostimulants is also vital. As a long-term strategy, selective breeding for disease resistance is of key significance in preventing viral diseases in brackishwater aquaculture.