Dietary natural components emerged as key supplements for protecting fish against aquatic pollutants. In this study, we explored the efficacy of thymol supplementation (1 or 2 g/kg diet) in alleviating the toxic effects induced by exposing Oreochromis niloticus to zinc oxide nanoparticles (ZON) at 1.14 mg/L (equivalent to 1/5th LC50) for 28 days. Relative to control, the fish treated with ZON showed a reduction in the survival rate and growth performance, with the highest zinc residues detected in the liver, followed by muscle and, finally, gills. Alterations were recorded in serum biochemistry reflecting ZON’s hepatotoxicity and nephrotoxicity, along with reductions in growth hormone, T3 (triiodothyronine) and T4 (thyroxine). The intestinal amylase, lipase, and protease activities were suppressed with increases in the intestinal Aeromonas and total bacterial counts in the ZON-exposed group. Elevated tissue levels of MDA (malondialdehyde) and PC (protein carbonyl) content, with depletion of the SOD (superoxide dismutase) and CAT (catalase) activities, and GSH (reduced glutathione) levels were also noted in ZON-exposed group, in addition to upregulation of the hepatic HSP-70 (heat shock protein- 70), Caspase-3, and P53 (Tumor suppressor protein). Moreover, histomorphological changes were recorded in the ZON-treated fish’s liver and gills. Conversely, concomitant administration of thymol with ZON blunted these toxic impacts in a dose-specific manner. Based on these data, thymol could be an effective dietary supplement for stressed fish. This investigation emphasizes the toxic impacts of ZON (1/5th LC50 =1.14 mg/L, for 28 days) in O. niloticus on growth performance, digestive enzymes, liver and kidney functions, as well as the hormonal profile. These effects are likely to be mediated via triggering oxidative stress, depletion of the cellular antioxidants, and overexpression of stress-related genes. In response to dietary thymol supplementation (1 or 2 g/kg), fish could avoid restricted growth performance, hepatorenal toxicity, and oxidative tissue injuries, evident by reducing MDA and PC content to near control levels. This coincided with augmentations of the antioxidant mechanisms and downregulation of stress-related genes, particularly with the higher dose (2 g/kg). The histomorphological investigation provided complementary evidence for the protective effect of thymol on liver and gill tissues. Taken together, our findings suggest the potential role of thymol as a protective supplement versus ZON toxicity in fish. However, more investigations are required to address further the mechanisms implicating its protective action