This study evaluated the effectiveness of BetaPlus® combined with isomalto-oligosaccharide (IMO) in Caspian brown trout, Salmo trutta caspius (Kessler), fingerlings. A total of 120 Caspian brown trout (8.75 ± 0.03 g) were fed in two treatments, including the control diet and the synbiotic diet (0.1% BetaPlus® + 0.2% IMO) in three replicates per treatment for seven weeks. The growth indices (final weight, weight gain, average daily growth, specific growth rate, feed efficiency, and protein efficiency ratio) exhibited significant improvement in the fish fed the synbiotic diet (P < 0.05). The highest ash crude protein, and crude fiber, as well as the lowest crude lipid, dry matter, and carbohydrate detected in the carcass of fish treated with the synbiotic were significant (P < 0.05). In addition, the fish fed the synbiotic diet showed significantly higher gut trypsin activity and trypsin:chymotrypsin ratio, as well as serum superoxide dismutase activity (P < 0.05). Thus, BetaPlus® in combination with IMO can effectively lead to a considerable increase in functional factors, as well as gut proteases and serum antioxidant indicators in S. trutta caspius fingerlings.