The effects of a dietary probiotic mixture containing Lactococcus (Lc.) lactis BFE920 isolated from bean sprout and autochthonous Lactobacillus (Lb.) plantarum FGL0001 originally isolated from the hindgut of olive flounder (Paralichthys olivaceus) were investigated for the purpose of improving the probiotic effects of Lc. lactis BFE920 on the olive flounder. The immunostimulatory, disease protective, and weight gain effects of Lc. lactis BFE920 were significantly improved when olive flounder (average weight 37.5 ± 1.26 g) were fed the probiotic mixture (log10 7.0 CFU each/g feed pellet) for 30 days. Flounder fed the mixture showed improved skin mucus lysozyme activity and phagocytic activity of innate immune cells compared to flounder fed a single probiotic agent or a control diet. While the levels of neutrophil activity in flounder fed the single probiotic agent or the mixture were similar, they were significantly higher than levels in a control group. Additionally, probiotic-fed flounder showed significantly increased expressions of IL-6, IL-8, and TNF-α in the intestine compared to the control group. Following a 30-day period of being fed probiotics or a control diet, the olive flounder were challenged with an i.p. injection of Streptococcus iniae (log10 6.0 CFU/fish). The groups fed the mixed probiotics, Lc. lactis BFE920, Lb. plantarum FGL0001, and the control diet had survival rates of 55%, 45%, 35%, and 20%, respectively. Flounder fed the probiotic mixture gained 38.1 ± 2.8% more body weight compared to flounder fed the control diet during the 30-day study period. These data strongly suggest that a mixture of Lc. lactis BFE920 and Lb. plantarum FGL0001 may serve as an immunostimulating feed additive useful for disease protection in the fish farming industry.