Hemorrhagic disease caused by bacteria has been one of significant problem in cultured mudskipper (Pseudapocryptes elongatus) in the Mekong Delta of Vietnam. Usual clinical signs of  disease include haemorrhage in skin, fin and anal with high mortality and widespread. These clinical signs are easy to spot for the layman. However, the observation of the clinical signs from a macroscopic perspective is insufficient to determine the species of bacteria responsible for the disease. These pieces of information are crucial to be collected in order to anticipate the utilization of prevention and treatment.  Therefore, diseased specimens were collected at a farm practicing intensive culture of mudskipper in Bac Lieu and Soc Trang provinces, where culture of mudskipper are comonly practised. Microscopic observation of fresh smear of blood, liver, kidney and spleen from these specimens revealed small cocci, gram positive bacterial cells. Bacteria isolates from brain and head kidney were recovered on brain heart agar and were analyzed as Gram positive, non-motile and oxidase negative and they were identified as Streptococus disgalactiae using a combination of conventional biochemical test, API 20 strep system, PCR and gene sequencing. Histopathological examination of diseased specimens showed a typical sign of bacterial necrosis in kidney, spleen and liver. Challenge experiments using injection method showed that they can cause the observed disease signs with the LD₅₀ value of about 4.25 x 10⁴ CFU/m. It is the first report of Streptococcus disgalactiae outbreak in mudskipper in Vietnam.