| Sumari: | Bacterial infection is frequently observed in disease outbreaks of aquatic animals, making it of significance to isolate and identify the bacterial pathogens. In this study, diseased individuals of <i>Crassostrea gigas</i> were sampled from the nearshore area in Zhanjiang, Guangdong in May 2023. Culturable bacteria were isolated from the diseased tissue and a pathogenic strain labeled as H27 was screened through a hemolysis test and bacterial challenge experiments. Morphological characterization, 16S rRNA gene sequence-based molecular identification and biochemical tests showed that strain H27 belonged to the genus of <i>Pseudoalteromonas</i>, a dominant genus in the diseased tissue of <i>C. gigas</i> revealed by bacterial community structure analysis. The clinical signs originally observed in naturally diseased <i>C. gigas</i> were reproduced in strain H27-challenged adults, both with the red mantle and adductor. Histopathological analysis was further performed on the diseased tissues of the latter, which showed a significantly increased accumulation of pigment granules in the cytoplasm of the diseased mantle as well as enlarged muscle fiber distances in the diseased adductor. In addition, strain H27 was re-isolated from tissues of the moribund <i>C. gigas</i> after bacterial challenge, indicating the fulfillment of Koch’s postulate. Our results help to enrich the knowledge of <i>C. gigas</i> diseases, possibly contributing to disease prevention and control.
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