Imidacloprid is among the most widely used insecticides and may contaminate surface waters, yet there is limited information available on their effects on fish. In the present study, juvenile catfish (Clarias gariepinus) were exposed to 100, 130, 160, and 190 mg/L of imidacloprid for 96 hours, and the effects on mortality, behavior, hepatic and endocrine functions, oxidative stress, and tumor and inflammatory responses were investigated. The 96 hours LC50 was 166.60 (143.38-193.59 95% confidence intervals) mg/L with mortality recorded at concentrations ≥ 130 mg/L. Exposure to imidacloprid induced behavioral alterations and clinical symptoms including gulping, hypoactivity, abnormal surface distribution, loss of buoyancy, and excessive mucus secretion. Furthermore, amylase, lipase, conjugate bilirubin, carbohydrate antigen 19 − 9 (CA 19 − 9), thyroid stimulating hormone (TSH), and glutathione peroxidase (GPx) activity were significantly increased, while significant reductions were observed in triiodothyronine (T3) and superoxide dismutase (SOD) activity. Non-significant changes were observed in aspartate aminotransferase (AST), alanine aminotransferase (ALT), total bilirubin, 17β- estradiol (E2), thyroxine (T4), C-reactive protein (CRP), glutathione S-transferases (GST), total antioxidant capacity (TAC) and catalase (CAT) activity. Our results demonstrate that imidacloprid causes behavioral, hepatic, and endocrine toxicity in catfish as well as oxidative stress and tumor marker elevation. Our findings show that short-term exposure to imidacloprid triggers biochemical and physiological responses in juvenile catfish.