The effect of dietary supplementation with a synbiotic mixture of galacto-oligosaccharides (GOS) and Bacillus spp. was examined in Caspian salmon, Salmo trutta caspius (Kessler, 1877) fingerlings. Caspian salmon fed with the synbiotic diet had significantly higher weight gain rate, protein efficiency ratio, and survival rate, as well as lower feed conversion ratio, compared to the control group (P < 0.05). The serum protein, albumin, globulin, and lactate dehydrogenase levels of the fish fed with the synbiotic diet were significantly higher than the control group (P < 0.05), while the serum alkaline phosphatase levels were significantly lower (P < 0.05). The activities of the innate immune response parameters, including lysozyme, superoxide dismutase, and catalase were significantly higher in the Caspian salmon fed with the synbiotic diet (P < 0.05). The gut microbiota of the Caspian salmon fed with the synbiotic diet contained significantly elevated total viable aerobic bacterial counts (TVABCs), lactic acid bacteria (LAB) levels, and LAB/TVABCs ratio (P < 0.05). Additionally, the gut activities of amylase, trypsin, and chymotrypsin in the gut, as well as the trypsin/chymotrypsin ratio, were significantly increased in the fish that received the synbiotic diet (P < 0.05). In conclusion, the combined GOS and Bacillus spp. supplement positively affected the growth, survival rate, immunobiochemical parameters, digestive activity, and beneficial microbial density in the gut of Caspian salmon fingerlings.