This work aimed to appraise the role of dietary seaweed Sargassum thunbergii supplementation in the growth performance, body composition, and immune responsiveness of turbot (Scophthalmus maximus). A basal diet (without supplementation) was supplemented with increasing seaweed powder content to formulate five groups of experimental diets containing 0, 2.5%, 5%, 7.5% and 10% of seaweed powder (denoted as ST0, ST2.5, ST5, ST7.5, ST10 groups). Turbot juveniles (initial weight 66.45 ±0.52 g) were fed experimental diets for 8 weeks. Results showed that weight gain rate (W_GR) and specific growth rate (S_GR) tended to decrease with the increasing dietary content of seaweed powder, however, there was no significant differences between ST2.5 and control group (P> 0.05), and the other groups were significantly lower than the control group (P <0.05). The daily feed intake (D_FI) showed a gradually decreasing trend, among which ST10 group was significantly lower than control group (P <0.05). Dietary seaweed supplementation had no effect on feed conversion ratio (F_CR), survival ratio (S_R), viscerosomatic index (V_SI) and hepatosomatic index (H_SI). Crude protein content of whole fish tended to increase gradually, and crude fat content increased firstly and then decreased, however, both had no significant differences among groups (P> 0.05). Crude ash content increased gradually (P<0.05), which was consistent with the trend of crude ash content of feedstuff. The concentration of serum low density lipoprotein cholesterol (LDL-C) showed a trend of first increase and then decrease (P<0.05), and reached the highest value at ST5 group. Serum acid phosphatase (ACP) and catalase (CAT) activity increased first and then decreased, and also reached the highest value at ST5 group. Under the present experimental conditions, the addition of 2.5% S. thunbergii powder had no significant effect on juvenile turbot growth. Adding 5% S. thunbergii powder significantly increased the non-specific immune capacity of juvenile turbot.