Farmed animals, including fish, often exhibit a pronounced individual variation in stress coping styles with proactive and reactive individuals differing in a variety of neuroendocrine and behavioural responses. In this study we disclosed that individual differences in cortisol responsiveness after a restraining test were predictive of aggressive behaviour in gilthead seabream Sparus aurata, one of the most important Mediterranean farmed fish. Seabream juveniles (n = 24, initial weight: 49.3 ± 7.3 g) were exposed to a restraining test that consisted of keeping each fish in an emerged net for three minutes. Afterwards fish returned to their home tank and 30 min after were rapidly caught and anaesthetised for blood sampling. Blood was collected from the caudal vein and analysed for cortisol (radioimmunoassay). After 3 months the same individuals were exposed to an aggression test: fish were allowed to interact with a naïve fish of similar size (max 10% weight difference) for 20 min. Aggressive behaviour was determined as follows: latency to start chasing (time taken until the first chase; chase was defined as a sudden change in swimming direction and speed as a response to an approach by the opponent); number of chases; latency to fighting (time taken until the first fight; fight was defined when fish engaged in a circle movement around each other) and number of fights. Results showed that individuals exhibiting lower cortisol responsiveness after a restraining test were more aggressive (cortisol vs number of chases: rs = −0.479, p = 0.021) In conclusion, seabream juveniles exhibited pronounced individual differences in cortisol responsiveness and aggression that are interrelated and likely to be distinctive traits of coping styles.