Frequent mortality was observed in the larval rearing facility of marine clownfish, Amphiprion sebae Bleeker. Mortality resulted in 80% loss of larval stock. Moribund larvae showed clinical signs typical of viral nervous necrosis, such as uncoordinated, corkscrew-like swimming behaviour, hypersensitivity to stimuli, darkening of body and assembly into large groups, similar to bunches of grapes. The aetiology of the disease was confirmed by gross observation of clinical signs, histopathology and molecular diagnosis. Histological studies revealed severe vacuolation in the brain and in the bipolar and ganglion layers of the eye. Molecular diagnosis by reverse transcription-polymerase chain reaction (RT-PCR) specific to piscine nodavirus yielded a positive result. The partial nucleotide sequences of the PCR-amplified fragment were 97-98% similar to other betanodavirus isolates reported globally and more closely aligned with red-spotted grouper nervous necrosis virus (RGNNV). This is the first report of susceptibility of clownfish, A. sebae, to betanodavirus and the presence of the RGNNV in India.