This study was conducted to investigate the utilization of Sargassum meal in the diet on juvenile ayu (Plecoglossus altivelis) reared in seawater. White fish meal and wheat flour were used as the dietary protein and carbohydrate sources in the control diet. Wheat flour in the control diet was replaced with $5\%$ and $10\%$ Sargassum meal. Three replicate groups of fish average weighing 4.0 g were fed one of three isonitrogenous$(45\%)$ and isocaloric (14.5 MJ /kg diet) diets for 7 weeks. Survival of all groups were above $80\%$. Weight gain, feed efficiency and protein efficiency ratio were not significantly affected by dietary Sargassum meal levels (P>0.05). There were no significant differences (P>0.05) in moisture, crude protein, crude lipid, crude ash and fatty acid compositions of whole-body fish among groups. It is concluded that Sargassum meal could be used as a dietary additive or alternative low-cost dietary ingredient up to $10\%$ for juvenile ayu reared in seawater.