This study was designated to look into the toxicological effects of long-term exposure of African catfish (Clarias gariepinus) to a heavy metal (copper) on the growth, hematology, immune-antioxidant functions and their regulating genes, and resistance to Aeromonas sobria. Furthermore, the efficiency of dietary zinc oxide nano­ particles (ZNPs) to ameliorate copper toxicity was assessed. Fish (n=300: average body weight: 90.56±0.20 g) were equally allocated into six groups in five replicates (10 fish/replicate; 50 fish/ group) for 60 days at a water temperature of 25 ± 1.2 ◦C. Three diets were formulated with varying ZNPs levels: control (0 mg kg− 1), ZNPs20 (20 mg kg− 1), and ZNPs30 (30 mg kg− 1), with actual zinc concentrations of 45, 65, and 75 mg kg− 1, respectively. The experimental groups were named, control, ZNPs20, and ZNPs30 groups which were fed on the control, ZNPs20, and ZNPs30 diets, respectively without copper exposure. Additionally, the CuCL2, CuCL2+ZNPs20, and CuCL2+ZNPs30 groups were received the same feeding regime as the control, ZNPs20, and ZNPs30 groups, respectively and exposed to copper chloride, CuCL2 (10 mg L− 1). At the end of the exposure trial (60 days), all experimental groups were challenged with 0.2 mL of A. sobria (1×108 CFU mL− 1) and the mortalities were recorded daily for additional 7 days. The results displayed that copper exposure resulted in lower growth metrics and survivability. The final body weight, weight gain, feed intake, and survivability were reduced in the CuCL2 group by 39.29%, 23.79%, 63.37%, and 20%, respectively with increasing the feed conversion ratio by 52.13% compared to the control group. In addition, reduced hematological (red blood cells (58.40%), hemoglobin (48.11%), packed cell volume (40.59%), white blood cells (46.85%), total proteins (43.12%), albumin (41.98%), and globulin (44.23%)), immune (lysozymes (59.74%), complement 3 (41.89%), nitric oxide (57.14%), and phagocytic activity (38.75%)), and antioxidant parameters (glutathione peroxidase (82.95%), catalase (74.08%), and superoxide dismutase (48.59%)), as well as higher oxidative (malondialdehyde) (200.30%) indicator were observed in the CuCL2 group compared to the control group. A down-regulation of the antioxidant (glutathione peroxidase-1) and immune (lysozyme-G, lysozyme-C, transforming growth factor-beta, and myeloperoxidase) genes with up-regulation of the inflammatory cytokines (interleukin-1ß, interleukin-8, tumor necrosis factoralpha, nuclear factor kappa β, and Toll-like receptor 5) was noticed in the CuCL2 group. In addition, the sur­ vivability of the CuCL2 group during A. sobria challenge was reduced (25%) compared to the control group. Noticeably, ZNPs diets modulated the growth, survival, and antioxidant/immune functions and their associated genes of the copper-exposed fish.