A 60-day experiment was organized to determine the optimum stocking density of Siganus rivulatus under biofloc system conditions during the nursery stage. The densities of fish samples with 400, 600 and 800fry/ m³ were subject to evaluation. Nine tanks made of plastic with dimensions of 48 X 35 X 20cm were used for the experiment. The stocked fish fries had an average starting body weight of 0.76 ± 0.03g. All fries were fed a commercial diet containing 40% crude protein (CP) at a rate of 4% of the body weight for six days per week. The highest weight gain (WG), average daily gain (ADG), specific growth rate (SGR), and protein efficiency ratio (PER) were recorded for the fish group stocked at the density of 600fry/ m³ (2.3, 0.04, 2.32, and 3.27, respectively). In addition, this group showed the best feed conversion ratio (FCR), with a value of 1.22. For histological sections, the histopathological alterations in the gills, liver, and intestines were recorded for the fish group stocked at a density of 800fry/ m³. Consequently, it could be concluded that, under the biofloc system, the stocking of S. rivulatus at medium density (600fry/ m³) gives the best growth performance, without affecting the welfare of fish throughout the nursery stage