Vibrio harveyi is a motile marine bacterium frequently linked to fish and shellfish diseases in aquaculture, particularly vibriosis, which leads to high mortality rates in marine species. This pathogen also poses a public health concern due to its potential to contaminate seafood and cause gastrointestinal infections in humans. The growing antibiotic resistance underscores the importance of effective monitoring and management strategies to safeguard both aquatic life and human health. In this study, 120 specimens of the European seabass (Dicentrachus labrax) and water waere collected from mariculture floating net cages during episodes of widespread fish mortality in marine fish farms. The moribund fish European seabass were thoroughly examined to assess clinical, post-mortem, and bacteriological aspects of the disease, thus providing evidence for the occurrence of a septicemic-hemorrhagic bacterial disease. Identification of the bacterium included morphological analysis using the API 20E system, 16S rRNA characterization through PCR and gene sequencing, and testing against Vibriostat O/129. Antibacterial susceptibility tests and histopathological examinations revealed an immune cell infiltration in the interstitial tissue, indicating a defensive response to infection. In addition, Vibrio harveyi strain could resist several antibiotics, resulting in difficulties in fish treatment. The study aimed to enhance understanding of the disease affecting seabass and explore potential therapeutic interventions to mitigate its impact.