Origanum vulgare is a well-known medicinal plant that has been used since ancient times as an additive in foods and cosmetic preparations. The possible application of O. vulgare extracts in fish was assessed by using gilthead seabream (Sparus aurata) as a marine fish model due to its importance in aquaculture. The in vitro effects of aqueous and ethanolic leaf extracts of O. vulgare were tested in order to observe any immunostimulant, cytotoxic, bactericidal or antioxidant properties. The results showed that medium or high concentration of aqueous extracts and low concentrations of ethanolic extract, increased head kidney leucocyte activities as well as the number of SAF-1cells. However, moderate to high concentrations of ethanolic extracts decreased both leucocyte activities and the number of viable SAF-1cells, suggesting some possible toxic effect towards them. Only the highest concentration of the aqueous extract and medium to high concentrations of the ethanolic extracts showed cytotoxic activity against the tumor PLHC-1cell line. Bactericidal activity was only detected against Vibrio harveyi, V. anguillarum and Photobacterium damselae when using the highest concentration of aqueous extract and moderate to high concentrations of ethanolic extract. Finally, both plant extracts presented antioxidant activity particularly the aqueous extract. Overall, the results suggest that both extracts (when used at the appropriate concentration) have immunostimulant, cytotoxic, bactericidal and antioxidant properties, making O. vulgare an interesting candidate for incorporation as additive in functional diets for farmed fish.