To identify and quantify the effects of a combination of dietary 1 × 108 CFU/g Lactococcus lactis subsp. lactis I2 (LI2) and 0.1% β-glucooligosaccharides (BGO) on the growth and immunity of olive flounder (Paralichthys olivaceus), a feeding experiment was conducted. Flounder (14 ± 0.5 g) were divided into two groups and fed control and synbiotic feeds for 8 weeks. Investigations were carried out on growth and feed utilization, innate immunity, serum biochemical parameters, intestinal lactic acid bacterial (LAB) viability, microvillus length, and changes in the expression levels of genes encoding pro-inflammatory cytokines (tumor necrosis factor [TNF]-α, interleukin [IL]-1β, and IL-6). Results demonstrated the synbiotic diet had significantly better (p < 0.05) responses in terms of weight gain and specific growth rate, three innate immune parameters (respiratory burst, serum lysozyme, and superoxide dismutase), intestinal LAB viability, and the relative TNF-α expression level (p < 0.05). Moreover, after challenge with Streptococcus iniae (1 × 108 CFU/ml), the synbiotically fed group exhibited significantly higher (p < 0.05) protection against streptococcosis, validating the observed changes in immune parameters and induction of the cytokine-encoding gene. Therefore, according to the results of the present study, synbiotic feed (LI2 + BGO) increased growth, modulated innate immune parameters and protected olive flounder against streptococcosis.