This study examined how European seabass, Dicentrarchus labrax, juveniles are affected by heat stress in several ways, including growth biometrics, stress indicators, oxidative stress biomarkers, and histopathological changes. Our research aims to gain a better understanding of the impact of thermal stress on these parameters. Hence, European seabass juveniles (30-32 g) were exposed to temperatures of 20 °C, 23 °C, 26 °C, 29 °C, and 31 °C using a 28-day bioassay. It was noted that the fish showed better performance indices at 23 °C and 26 °C. However, fish reared at 20 °C showed intermediate growth, while the fish reared at 31 °C displayed poor performance with low survival rates. As the water temperature increased from 20 to 31 °C, the levels of glucose, cortisol, aspartate aminotransferase, and alanine aminotransferase in the fish blood also increased, suggesting that the fish were under stress. Furthermore, activities of superoxide dismutase (SOD) and catalase (CAT), as well as levels of malondialdehyde, increased significantly (P <
0.05) with the rise in the rearing temperature, particularly at 31 °C. This suggested that European seabass juveniles experienced oxidative stress. Additionally, the mRNA expression of SOD and CAT genes was upregulated at 31 °C compared to those reared at 23-26 °C. This high upregulation of both genes led to an increase in the secretion of SOD and CAT. Juveniles of European seabass raised in 31 °C for 28 days showed significant damage in the histological structure of their kidney, liver, and gills. In addition to fusion and blood congestion of secondary lamellae, the fish in this treatment (31 °C) displayed edema, epithelial lifting, and blood congestion of the gill epithelium. After 28 days, fish cultivated at 31 °C had sinusoid dilatation, hyperemia, and nuclear hypertrophy in their liver tissues. Furthermore, hyperemia, tubular necrosis, and severe glomerular congestion were observed in fish raised in water temperatures as high as 31 °C for 28 days. This study recommends farming European seabass at 23 °C and 26 °C, which were the optimum temperatures. By global warming due to climatic changes, water temperature may reach up to 31 °C or more, which will cause adverse effects on fish performance and increase the oxidative stress.