Supplementation of seaweed to fish diets has been widely reported to improve growth and immunity. This study was conducted to investigate the effect of sulfated polysaccharide extract from the green alga Ulva intestinalis and the red alga Gracilariopsis persica as a dietary supplement on growth performance, fatty acid profile, stress response, antioxidant enzyme activity, and nonspecific immune responses in rainbow trout (Oncorhynchus mykiss) (15.74 ± 0.48 g). The five experimental diets include control (without sulfated polysaccharide), 0.5SPG (0.5 g kg⁻¹ sulfated polysaccharide from G. persica), 1.5SPG (1.5 g kg⁻¹ sulfated polysaccharide from G. persica), 0.5SPU (0.5 g kg⁻¹ sulfated polysaccharide from U. intestinalis), and 1.5SPU (1.5 g kg⁻¹ sulfated polysaccharide from U. intestinalis) were formulated and fed to fish for 8 weeks. Results of growth trial indicated a significant enhancement of growth performance of fish fed 1.5SPG, 0.5SPU, and 1.5SPU as compared with control. Although there was no significant difference in the fatty acid profile of muscle, fish fed the 1.5SPG diet showed higher protein and lower lipid contents than the control group. While the lysozyme and complement activity of serum was significantly enhanced in fish fed 1.5SPG and 1.5 SPU diets, superoxide dismutase in these treatments was lower than the control group. Furthermore, the amount of cortisol in individuals fed experimental diets after 3-h confinement stress was lower than fish fed the control diet. To conclude, in consideration of all factors, we suggest the potential benefits of using 1.5SPG diet in the aquaculture industry as a diet supplementation.