<p>This research examined the impact of dietary garlic powder supplementation, a medicinal plant that contains antioxidant and bioactive compounds on growth performance, hematobiochemical indices, immune responses, and mRNA expression of stinging catfish (<i>Heteropneustes fossilis</i>). A total of 480 fingerlings were randomly allocated into four experimental groups, each consisting of three replicates. The control group was fed a basal diet, whereas the experimental groups were fed diets supplemented with garlic powder at 5, 10, and 20&#xa0;g/kg (designated T1, T2, and T3, respectively) for 90 days. The addition of 10&#xa0;g/kg garlic powder (T2) significantly improved growth, feed utilization efficiency, biometric parameters, and hematological profiles compared with other groups (<i>p</i> &lt; 0.05). Blood biochemical analysis indicated reduced glucose levels and elevated serum protein concentrations in T1 and T2 fish. Lysozyme activity was significantly increased in these groups. Furthermore, hepatic enzyme levels were significantly lower in T2 fish compared to the control and other treatments. The expression of stress-related genes (<i>HSP70</i> and <i>HSC70</i>) was significantly decreased, whereas the antioxidant genes (<i>SOD2</i> and <i>GPx4</i>) showed a notable increase in the hepatic tissue of T2 fish (<i>p</i> &lt; 0.05). The dietary inclusion of 10&#xa0;g/kg <i>A. sativum</i> enhanced growth, immunity, and antioxidant gene expression, underscoring its potential as a natural growth promoter and phytogenic agent for physiological enhancement and stress mitigation in aquaculture.</p>