Extreme water temperatures related to global climate change represent a significant challenge in terms of fish welfare and aquaculture productivity. Understanding the effect of more frequent and extended extreme temperature events on physiological responses, growth performance and other production-relevant parameters of major aquaculture species are essential for developing suitable mitigation methods and assuring future production. Fish were exposed to 8 °C, 16 °C, 24 °C and 32 °C to evaluate the extreme ambient temperatures effect on growth performance, physiological, biochemical and molecular responses of juvenile European seabass, Dicentrarchus labrax. Fish exposed to 8 °C and 32 °C for 30 days exhibited significantly lower final weight, weight gain, specific growth rate, survival rate, and temperature growth coefficient (p < 0.05) compared to 16 °C and 24 °C. Hepatosomatic index (HSI), viscera somatic index (VSI), intestine somatic index (ISI) and spleen somatic index (SSI) were significantly (p < 0.05) lower in fish at 8 °C and 32 °C at day 30. Plasma [Na+] and [Cl−] ion concentrations were significantly lower (p < 0.05) in fish reared at 8 °C at day 10, 20 and 30. Plasma triglycerides, lactate, cortisol were significantly (p < 0.05) increased, while plasma glucose, protein and liver energy storage showed the inverse trend in 8 °C and 32 °C reared fish at day 10, 20 and 30. Heat shock proteins (HSP70) gene was significantly (p < 0.05) up-regulated in the dorsal muscle and kidney tissue of fish reared at 8 °C and 32 °C at day 10, 20 and 30. Whereas Interleukin 1β (IL-1β) gene exhibited a similar, but less regular expression with upregulation at day 10 across all four temperature treatments. Insulin growth factor 1 (Igf1) relative expression was decreased significantly in fish reared in 8 °C and 32 °C than in 16 °C and 24 °C at day 10, 20 and 30. European seabass exhibits significant physiological, biochemical and gene expression alterations and marked performance reduction during extreme temperatures of 8 °C and 32 °C. None of the repeatedly measured parameters in the current study indicated a capacity for compensation by physiological adaptation over periods of 10, 20 or 30 days.