Efforts are made to produce functional aquaculture diets capable of promoting fish growth and health while being sustainable at social, economic and environmental levels. One of the emerging threats in aquaculture has become the antibiotic resistance phenomena due to antimicrobial drugs. Therefore, functional feed additives of marine origin have been introduced globally as an alternative to fish antibiotics. Towards the selection of natural and sustainable resources of bioactive compounds, seaweed has been proven to be a source of valuable substances showing antibacterial activity. Aim of this study is to evaluate the inhibition of bacterial growth caused by an ethanolic extract of the macroalgae Ulva lactuca against important aquaculture bacterial strains. Vibrio anguillarum O1, Photobacterium damselae sub. piscicida and Tenacibaculum maritimum, were incubated for 48 hours in sterile Brain Heart Infusion Broth and tested for resistance to the extract using broth cultures. The algal extract successfully inhibited the growth of all strains. The optimum inhibition was achieved for VAO1 and PHDP by the undiluted and 1:1 diluted extract, while for TMAR, by the 1:3 dilution as well. The antibacterial activity that the extract provided is considered to be the result of the containing bioactive compounds of the algal strain, such as polysaccharides, carotenoids and phenolic compounds.