Soy protein concentrate (SPC) rarely replaces fishmeal (FM) completely in marine finfish diets without eliciting adverse effects on fish performances. The inclusion of brown seaweed (Sargassum fulvellum) could alleviate the negative effects linked to high SPC dietary inclusion. In the current study; the dietary effects of S. fulvellum inclusion in diets for Japanese flounder (Paralichthys olivaceus) in which FM was partially replaced with SPC were evaluated. A FM-based positive control (Con+) diet with FM protein partially replaced with SPC protein (Con-) was supplemented with S. fulvellum at levels of 30; 60 and 90 g/kg to formulate SF3; SF6 and SF9 diets; respectively. The five diets were fed to fish (0.52 g) in triplicate groups up to apparent satiety for 56 days. The dietary brown seaweed significantly increased (p < .05) growth performance; showing significantly higher per cent weight gain (p < .05) and specific growth rate in fish fed SF6 and SF9 compared to other treatments. Seaweed inclusion also significantly increased (p < .05) feed intake and feed utilization performances. Fish fed Con- diet exhibited significantly lower (p < .05) growth and feed utilization performances. The highest muscular and hepatic total lipids were observed in fish fed Con+. Seaweed inclusion significantly increased and decreased (p < .05) muscular and hepatic total lipid contents; respectively; when compared to Con- diet. The muscle fatty acids were not significantly (p > .05) affected by dietary S. fulvellum. Hepatic total n-3 and total n-3 + total n-6 fatty acids were significantly higher (p < .05) in supplemented groups compared to Con+ and Con-. Finally; S. fulvellum inclusion at 60 g/kg is found to be beneficial for increasing growth; feed utilization and some fatty acid restoration in SPC-incorporated Japanese flounder fry diets.