The objective of this study was to evaluate the effect of dietary addition of bentonite (BNT) clay as a toxin adsorbent for diazinon (DZ) in Nile tilapia. Fish were randomly divided into five groups; the first group (G1) was fed on a basal diet and serve as a control group; the second group (G2) was exposed to DZ at a dose of 0.39 mg/ L (1/20 of 96 h LC50); the third group (G3) was fed on a basal diet supplemented with BNT (5 g/kg diet) and exposed to DZ 0.39 mg/ L; the fourth group (G4) was exposed to DZ at a dose of 0.78 mg/ L (1/10 of 96 h LC50), and the fifth group (G5) was fed on a basal diet supplemented with BNT (5 g/kg diet), and exposed to DZ at a dose of 0.78 mg / L for 90 days. The liver and kidney function indicators, acetylcholinesterase (AchE) levels, antioxidant capacity, oxidative stress, DZ residues, and histopathological changes were assessed. Results showed that DZ-exposed fish exhibited a significant elevation (p < 0.05) in the liver and kidney function indicators as aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP), urea, and creatinine. A significant decrease (p < 0.05) in serum levels of acetylcholinesterase (AchE), catalase (CAT), copper zinc superoxide dismutase (Cu Zn-SOD), and reduced glutathione (GSH), whereas there was a significant enhancement (p < 0.05) in the oxidative stress marker malondialdehyde (MDA), accompanied by a significant elevation (p < 0.05) in DZ accumulation in the liver, kidney, and muscles after 45 and 90-days of DZ exposure. The more severe clinical signs and postmortem lesions were recorded in G2 of DZ-intoxicated fish at both 45 and 90- day intervals. The signs severity diminished in BNT-treated groups (G3 and G5). The toxicity of DZ was evidenced by histopathological alterations in the liver, kidneys, spleen, and gills. Notably, the addition of BNT to the diet of DZ- intoxicated fish can mitigate the disorders in most of the serum biochemical parameters, clinical signs and, postmortem lesions as well as tissue architecture of Nile tilapia.