The present investigation evaluated the negative impacts of an aflatoxin-contaminated diet on the growth performance, serum and tissue biochemistry, gene expression of some immune-related genes in the liver, and pathological lesions in some organs of Nile tilapia. Moreover, the ameliorative effects of Moringa oleifera against fish aflatoxicosis were evaluated. Fish were allocated into four groups with the M. oleifera inclusion and/or aflatoxin contamination. The groups are the first group (negative control group) fed on a basal diet without any additives, the second group (positive control group) fed on a diet containing 1 mg aflatoxin B1/kg diet. The third and fourth groups fed the contaminated diets supplemented with 0.5% and 1% M. oleifera seeds extract, respectively. Results showed the adverse effects of aflatoxin on the growth performance, serum biochemistry, liver tissue biochemistry, and gene expression and pathology of different organs of Nile tilapia. These adverse effects were ameliorated by the M. oleifera supplementation at 0.5%. In conclusion, the improvement of the biochemical and innate immune response parameters of tilapia fed diets contaminated with aflatoxin revealed the immunomodulatory role of the M. oleifera in the tilapia diet.