In aquaculture, phytobiotics are used as growth promoters, immunomodulators, antioxidants, and stress mitigators in several fish species. Herein, we assess the impact of Moringa oleifera seeds petroleum ether extracts (MSE) on growth, hematological, and biochemical responses in whisker catfish (Mystus gulio). A total of 120 M. gulio fingerlings (mean weight 2.09 ± 0.16 g) were randomly distributed in four different experimental groups. Fish were fed to satiation with the experimental diets [T1 (control, 0% MSE), T2 (0.05% MSE), T3 (0.10% MSE), and T4 (0.15% MSE)] for 60 days. Results demonstrated that fingerlings fed with 0.05 and 0.10% MSE-supplemented diet significantly enhanced weight gain, specific growth rate, protein efficiency ratio, hemato-immunological indices (erythrocyte count, leucocyte count, hemoglobin, globulin, serum protein, and respiratory burst activity) and survival compared to other groups (P <
0.05). However, at the higher dose of MSE (0.15%), a significant reduction in the above parameters was observed (P <
0.05). Feeding MSE at 0.05 and 0.10% significantly minimizes the stress responses (blood glucose, cortisol, and catalase). Dose-dependent reduction in serum cholesterol and high-density lipid and an increase in triglyceride were recorded (P <
0.05). Histopathological observation of liver tissue showed congestion of hepatic vein at 0.15% MSE supplementation. Overall, the study suggests that the dietary supplementation of MSE up to 0.10% improves growth, biochemical, and innate immune response in M. gulio fingerlings.