Carob leaves, the main residues of the carob tree, were investigated as a renewable and abundant source of bioactive compounds for fish aquaculture. Aqueous and ethanolic extracts obtained from carob leaves were characterized in terms of biochemical composition, antiradical and cytotoxic effects and immunostimulant and antibacterial activities. The ethanolic extract showed higher levels of total phenolics, flavonoids and condensed tannins and higher antioxidant activity than the aqueous extract. No significant immunostimulant effects were observed on gilthead seabream (Sparus aurata) head kidney leucocytes (viability, phagocytosis and respiratory burst activities and peroxidase content) after incubation for 24 h with different extracts. Furthermore, the ethanolic extracts used at 0.5, 0.75 and 1 mg mL^-1 and aqueous extracts at 1 gmL^-1 had a cytotoxic effect on PLHC-1 cells. When the bactericidal activity was tested against three fish pathogenic bacteria (Vibrio harveyi, Vibrio anguillarum and Photobacterium damselae) notable activity of the different extracts was detected against P. damselae at all three concentrations. A similar effect was demonstrated against V. haryeri when ethanolic extracts were used in the same range of concentrations. This work demonstrates interesting in vitro effects of carob leaf extracts and suggests it could be used as an alternative to chemical compounds with farmed fish. The concentration and nature of the extracts were very important in terms of any positive results.