Vibriosis caused by Vibrio harveyi is a significant disease in Asian sea bass, leading to massive mortality, reduced growth, and considerable economic losses in aquaculture. This study aimed to evaluate the efficacy of an oral vaccine containing inactivated Vibrio harveyi with nanochitosan as an adjuvant on the immunity and survival of Asian sea bass. A total of 180 fingerlings (average weight 10 g) were divided into three groups: a control group (non-vaccinated), a group vaccinated with nanochitosan-adjuvanted vaccine, and a group vaccinated without adjuvant. Oral vaccination was administered in two rounds (days 0 and 14), with each round consisting of three daily feedings. All groups were maintained under identical conditions for 12 weeks and fed daily at a rate of 4% of their body weight. On day 70, a pathogen challenge was conducted via intraperitoneal injection of live Vibrio harveyi, and mortality was recorded over 7 days. The group receiving the nanochitosan-adjuvanted vaccine exhibited a stronger immune response (p < 0.05), higher survival, and improved growth compared to the control group. This study is one of the first in Iran to use a native strain of Vibrio harveyi and oral nanochitosan as an adjuvant, representing key innovations. The findings highlight the potential of the nanochitosan-adjuvanted oral vaccine in controlling vibriosis and may pave the way for future research into developing effective oral vaccines for aquaculture.