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Bacteriocins are emerging as a viable alternative to antibiotics due to their ability to inhibit growth or kill antibiotic resistant pathogens. Herein, we evaluated the ability of the bacteriocin Garvicin KS (GarKS) produced by <i>Lactococcus garvieae</i> KS1546 isolated from cow milk to inhibit the growth of fish and foodborne bacterial pathogens. We found that GarKS inhibited the growth of five fish <i>L. garvieae</i> strains isolated from infected trout and eels. Among fish pathogens, GarKS inhibited the growth of <i>Streptococcus agalactiae</i> serotypes Ia and Ib, and <i>Aeromonas hydrophila</i> but did not inhibit the growth of <i>Edwardsiella tarda</i>. In addition, it inhibited the growth of <i>A. salmonicida</i> strain 6421 but not <i>A. salmonicida</i> strain 6422 and <i>Yersinia ruckeri</i>. There was no inhibition of three foodborne bacterial species, namely <i>Salmonella enterica</i>, <i>Klebsiella pneumoniae</i>, and <i>Escherichia coli</i>. In vitro cytotoxicity tests using different GarKS concentrations showed that the highest concentration of 33 µg/mL exhibited low cytotoxicity, while concentrations ≤3.3 µg/mL had no cytotoxicity on CHSE-214 and RTG-2 cells. In vivo tests showed that zebrafish larvae treated with 33 µg/mL and 3.3 µg/mL GarKS prior to challenge had 53% and 48% survival, respectively, while concentrations ≤0.33 µg/mL were nonprotective. Altogether, these data show that GarKS has a broad inhibitory spectrum against Gram positive and negative bacteria and that it has potential applications as a therapeutic agent for a wide range of bacterial pathogens. Thus, future studies should include clinical trials to test the efficacy of GarKS against various bacterial pathogens in farmed fish.