Nile tilapia (Oreochromis niloticus) is a major species in aquaculture due to its rapid growth and adaptability. However, intensive farming conditions expose these fish to stressors such as overcrowding and poor water quality, which can weaken their immune system and increase susceptibility to diseases. Ensuring strong immunity is crucial for maintaining the health and productivity of tilapia in aquaculture settings. Tryptophan, an essential amino acid, has been found to reduce stress and boost immune function by acting as a precursor to serotonin, a neurotransmitter involved in stress regulation. Supplementing fish diets with tryptophan can help alleviate stress-induced immunosuppression, thereby improving overall health and disease resistance. In this study, Nile tilapia was divided into four dietary groups:1) control feed (CCF), 2) tryptophan-supplemented feed (CTF), 3) cortisol-induced stress with control feed (SCF), and 4) cortisol-induced stress with tryptophan-supplemented feed (STF). The fish were monitored over an eight (8) -week period to evaluate the impact of dietary tryptophan on immune and physiological parameters. The results showed that tryptophan supplementation significantly enhanced lysozyme activity, indicating improved innate immune response, especially under stress conditions. Macrophage phagocytic activity, essential for pathogen defense, was also significantly higher in the tryptophan-supplemented groups. Additionally, the hepatosomatic index (HSI), which indicates liver health, improved in tryptophan-treated fish, suggesting enhanced stress tolerance and overall physiological condition. These findings demonstrate that tryptophan supplementation can effectively strengthen the immune system and reduce the negative effects of stress in Nile tilapia, making it a beneficial dietary addition in aquaculture practices.