This study aimed to find the optimal fermentation technology of medicinal plants (turmeric, Curcuma longa and green tea, Camellia sinensis) mixed with Lactobacillus brevis (FML) and the effects of FML on the growth performance, immune response, and anti-microbial ability of rainbow trout fingerlings. To optimize the fermentation process, response to surface methodology (RSM) was applied. The optimal fermentation conditions were 8% probiotic and 25-h fermentation time. Fish (n = 600; 9.37 ± 0.24 g initial weight) was subjected to a feeding experiment involving five different diets. These diets included a control group without any additives as well as groups supplemented with various amounts of FML: 0.2% (FML1), 0.5% (FML2), 0.8% (FML3), and 1% (FML4) for a period of 60 days. Afterwards, a subset of 15 fish was selected from each tank and exposed to Yersinia ruckeri for 14 days. After the feeding trial, the growth indices significantly increased in FML-added groups (p < 0.05). The protease activity markedly increased in all FML-treated groups (except for FML1) compared to the control group (p < 0.05). Moreover, the serum bactericidal activity, immunoglobulin, total protein (TP), and globulin levels increased in FML3 and FML4 groups compared to those receiving the control and FML1 diets (p < 0.05). The skin mucus alkaline phosphatase and serum TP levels significantly increased in the fish fed with FML3 and FML4 diets compared to other treatments (p < 0.05). The highest sivival rate was observed in FML4 (83.3%) followed by FML3 (66.7%) group. Overall, dietary FML, especially in FML4 diet, has a beneficial role to ameliorate growth rate, immune functions, and disease resistance against Y. ruckeri in rainbow trout.