This study investigates the effects of the red macroalgae Galaxaura oblongata diet supplementation on roach (Rutilus rutilus) fingerlings growth and gene expression related to immunity, the intestinal barrier, and antioxidant status. Roach fingerlings (2.26 ± 0.04 g) were fed a basic diet supplemented with three different percentages of G. oblongata powder: 0.25 % (G1), 0.50 % (G2), and 1.0 % (G3) over 8 weeks, with a control group (C) receiving no supplementation. The study found that growth parameters significantly increased in the G1 and G2 groups compared to the C and G3 groups (P <
0.05). Immune responses, measured by total immunoglobulin (Ig) and lysozyme activity, showed a significant increase in the whole-body extract of the G2 group (P <
0.05) and the skin mucus of all treated groups compared to the control (P <
0.05). G. oblongata supplementation did not significantly affect catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities in the whole-body extract, although GPx activity in skin mucus was significantly higher in the supplemented groups. Additionally, the gene expression of interleukin 1-beta (il-1β), occludin, gpx and sod, but not Toll-like receptor increased in G. oblongata treated groups. These results suggest that G. oblongata can serve as a beneficial feed additive in the culture of roach fingerlings, enhancing growth and immune function.