The immunostimulants have proven efficacy in aquaculture, enhancing survival, growth, and immune responses. This research aimed to determine the optimal dietary level of the plant-based immunostimulant, holy basil (Ocimum sanctum), for maximizing the growth performance and non-specific immunological response of Cirrhinus mrigala fingerlings. Five experimental diets (C, OS1, OS2, OS3, and OS4) were prepared, incorporating holy basil powder at rates of 0, 25, 30, 35, and 40g kg–1, respectively. The fingerlings were fed these diets at 4% of their body weight over a 90-day period. Analysis of the collected data revealed significant improvements in growth parameters within the OS3 (35g kg–1) treatment group. This group exhibited enhanced live weight gain, growth percent gain in body weight, daily growth in percent body weight, specific growth rate, gross conversion efficiency, protein efficiency ratio, apparent protein digestibility, and intestinal enzymatic activity. Furthermore, certain hematological and immunological indicators, including total erythrocyte count, total leukocyte count, serum protein levels, respiratory burst activity, phagocytic activity, lysozyme activity, and carcass composition (crude protein and fat), displayed remarkable results in the OS3 group compared to other treatments. Minor changes were observed in water quality metrics across all dietary groups, except for ammonia and phosphate levels, which were significantly lower in the OS3 (35g kg–1) group compared to others. In a separate 10-day experiment, fingerlings were exposed to the pathogenic bacteria Aeromonas hydrophila (MTCC-1739), with the OS3 (35g kg–1) group displaying the highest survival rate. In summary, this investigation demonstrated that fingerlings exhibited optimal growth, non-specific immunological responses, and the highest survival rate when fed a diet containing 35g kg–1 of holy basil powder. These findings emphasize the effectiveness of holy basil as an immunostimulant in aquaculture.