Solar ultraviolet B radiation (UVB) has been underestimated as a stressor in fish species that grow in confined cages, especially in species cultured in oligotrophic waters that receive high levels of UVB. In this context, the present study aims to assess UVB radiation's adverse effects on European seabass (Dicentrarchus labrax), which is a cultured species in the Mediterranean and the Red Sea. Seabass juveniles were exposed for six weeks to four experimental conditions that simulated natural underwater conditions: (1) high dose - UVB-H, 11.86 kJ m−2 d−1; (2) moderate dose - UVB-M, 6.28 kJ m−2 d−1; (3) low dose - UVB-L, 2.55 kJ m−2 d−1; and (4) no UVB exposure as the control. Specimens were collected after 3, 15 (short-term), and 43 (long-term) days and effects of exposure on growth, behavior, physiology, and immune system were investigated. Overall, data showed that short- and long-term exposure to UVB resulted in detrimental effects in European seabass, and some of these effects were cumulative and dose-dependent. High mortality (around 50%) was observed in the seabass juveniles after 3 days of exposure to the highest daily dose (UVB-H). No significant mortality was observed in UVB-M, UVB-L, or control throughout the experiment. Reduction in growth and changes in animal body condition indices were evident in UVB-M exposed fish after 15 and 43 days of exposure. The swimming activity was reduced in the UVB-M treatment, and most of the fish from this treatment tried to avoid the exposure while showing a stationary behavior with slow caudal and dorsal fins movements. A lack of appetite was observed during the experiment, and the reduced total protein, glucose, lactate, and total cholesterol plasmatic levels in UVB exposed fish reflected physiological impairment. Furthermore, several changes in the humoral parameters suggest that an immune system modulation can occur in seabass exposed to the three UVB treatments (UVB-L, UVB-M, UVB-H), and during the different exposure periods (3, 15, and 43 days). Overall, our data provide evidence that, in the case of confinement in cages, European seabass juveniles are sensitive to UVB exposure, especially in oligotrophic waters with high UV radiation incidence such as the Red Sea. Hence, seabass' growth, resistance to pathogens, and survival may be affected by UVB exposure. Further research is recommended to understand how fish growth and survival may be affected by future increases in underwater UVB and when combined with other environmental stressors.