Garlic (Allium sativum) has had an important dietary and medicinal role for centuries and has long been known to have broad antibacterial properties. Streptococcus iniae occurs naturally in marine and estuarine environments and is one of the major opportunistic pathogenic bacteria in wounded or unhealthy grouper. The purpose of this study was to investigate the in vitro antibacterial activity of garlic and the effect of garlic-supplemented diets on growth performance and resistance against S. iniae in orange-spotted grouper (Epinephelus coioides). The in vitro antibacterial activity of garlic against S. iniae was investigated with the agar well diffusion, minimum inhibitory concentration and minimum bactericidal concentration methods and showed a strong inhibitory effect. Fish were fed diets containing 0, 1.3 and 4% of garlic powder for 14 days. The cumulative mortality of the fish fed with the 1.3% garlic diet was significantly (p<0.01) lower than that of those fed with the 4% garlic diet and control diet following a challenge with S. iniae and had significantly (p<0.01) higher percent weight gain and feed efficiency. The results indicate that the inclusion of 1.3% garlic in the diet of orange-spotted grouper enhances feed efficiency, fish growth and resistance to infection by S. iniae.