The evolution of microorganism resistance to antibiotics has resulted in growing need for new antibacterial compounds. In the present study, the antibacterial activity of green tea (Camellia sinensis L.) seed and its secondary metabolite saponin were evaluated on five bacterial fish pathogens that cause diseases in rainbow trout (Oncorhynchus mykiss). In addition to this, possible histological effects were investigated on the vital organs of fish. The antibacterial activity of tea seed and saponin were determined using the agar diffusion method. Watered tea seed (WTS) powder and saponin used in this research as the feed additives demonstrated antibacterial activity against the five pathogens but statistically significant inhibition was observed only with Listonella anguillarum. Accordingly, significantly higher survival levels were observed in fish infected with L. anguillarum in WTS and saponin diet groups. However, organs of fish fed with WTS and saponin showed some nonvital symptoms such as hyperplasia and epithelial lifting in the gills and lipid droplets, intercellular edema and nuclear degeneration in liver. Results from this study confirmed the potential use of WTS as a source of antibacterial compounds or as a feed additive for prevention of disease against L. anguillarum infection in trout culture.