Probiotics including Bacillus spp. are gaining a lot of interest in fish diets due to their distinctive pharmacological and nutritional value. To look at the impact of dietary fortification of a mixture of Bacillus spp. (MBs), a 56-day feeding trial was applied to European seabass (Dicentrarchus labrax). The evaluation addressed the influence on growth, absorption, hematology, biochemical, and immune-antioxidant response including molecular level. The resistance to Vibrio parahaemolyticus was also assessed. Five treatments having 225 fish (45 fish/treatment; three triplicates/treatment; 15 fish/replicate) were randomly assigned (8.00 ± 0.30 g body weight). The MBs were included in the basal diet at 0 (control), 1(MBs1), 2 (MBs2); 3 (MBs3); and 4 (MBs4) g/kg diet and given to the fish for 56 days. The results displayed that MBs-supplemented diets notably augmented growth (weight gain and specific growth rate) and intestinal morphology (villous length and width). The highest results for weight gain, specific growth rate, villous length and width were in the MBs3 (1.32-, 1.16-, 1.68-, and 1.45-fold) and MBs4 (1.33-, 1.17-, 1.78-, and 1.54-fold) groups, respectively, relative to the control. Dietary MBs (1–4 g/kg) significantly improved (P < 0.05) hematological [white blood cells count (1.12-fold), hematocrit (1.01-, 1.01-, 1.02-, and 1.04-fold), and hemoglobin (1.02-, 1.03-, 1.05-, and 1.06-fold)] and biochemical indices [albumin (1.05-, 1.08-, 1.11-, and 1.08-fold), globulin (1.13-, 1.24-, 1.46-, and 1.48-fold), total protein (1.09-, 1.16-, 1.29-, and 1.29-fold), and triglycerides (1.00-, 1.00-, 1.00-, and 1.01-fold)]. The antioxidant indices as catalase (1.15-, 1.15-, 1.16-, and 1.16-fold), superoxide dismutase (SOD; 1.06-, 1.09-, 1.10-, and 1.09-fold), and glutathione peroxidase (GPx; 1.02-, 1.10-, 1.11-, and 1.10-fold) were enhanced by dietary MBs (1–4 g/kg), respectively, compared with the control. The aspartate (0.98-, 0.98-, 0.97-, and 0.97-fold) and alanine (0.96-, 0.93-, 0.89-, and 0.89-fold) aminotransferases, alkaline phosphatase (0.97-fold), glucose (0.96-, 0.95-, 0.93-, and 0.93-fold), cholesterol (0.88-fold), and malondialdehyde (0.92-, 0.90-, 0.91-, and 0.91-fold) were declined by dietary MBs supplementation (1–4 g/kg), respectively. Moreover, the expression of growth [insulin-like growth factor 1 (1.41-, 1.91-, 2.15-, and 2.65-fold) and growth hormone (1.67-, 1.97-, 2.25-, and 2.72-fold)] and immuno-antioxidant associated genes [complement3 (1.42-, 1.95-, 2.33-, and 2.52-fold), interleukin-1 beta (1.30-, 1.76-, 2.43, and 2.82-fold), SOD (1.40-, 1.86-, 2.47-, and 2.64-fold), and GPx (1.22-, 1.75-, 2.55-, and 2.75-fold)] were up-regulated in MBs-fed groups (1–4 g/kg), respectively. Post-challenge with V. parahaemolyticus, the MBs3 and MBs4 groups had the highest survival rate (70.00 and 70.00 %), followed by the MBs2 group (60.00 %) and MBs1 group (50.00 %) compared to control (30.00 %). Depending on the broken line regression approach, our study suggests using dietary MBs at an optimal level of 3 g/kg diet. From overall results, MBs (3 g/kg diet) are recommended in European seabass to promote growth rate, feed utilization, immune-antioxidant status, and V. parahaemolyticus resistance without any hazardous impact on liver functions. These advantageous gains may help to ensure a sustainable aquaculture.