In the current experiment the effects of brown seaweeds (Sargassum aquifolium, SA) on blood biochemical, antioxidant, innate immune, and mRNA gene expression responses of Tilapia (Oreochromis niloticus) under high stocking rate were examined. The fish were allocated into equal six experimental groups: low stocking density (LSD) (the fish reared at 10 g L−1 and fed an un-supplemented diet), High stocking density (HSD) (the fish reared at 20 g L−1 and fed an un-supplemented diet), HSD + SA50, HSD + SA100, HSD + SA150, and HSD + SA200 (the fish reared at 20 g L−1 and fed Sargassum-supplemented diet at level 50, 100, 150, 200 g kg−1, respectively). After 56 days of the feeding trial, the fish at the HSD treatment revealed remarkedly reduced growth and feed efficiency compared to those at the LSD treatment, but the HSD + SA50 and HSD + SA100 fish groups showed a marked enhancement in performance and efficiency of consumed diets comparable to the LSD treatment. Under overcrowded stress, the fish fed diets enriched with Sargassum significantly diminished ammonia releasing levels within the entire environment. Serum total protein, albumin, liver enzymes, and lipase levels had significantly improved in HSD + SA150 and HSD + SA200 fish groups compared to those in the HSD treatment. Superoxide dismutase, catalase, and glutathione peroxidase exhibited high values at HSD + SA200- and LSD-treated groups than the HSD-treated group. However, HSD suppressed all plasma antioxidant activity. The high stocking rate remarkedly recorded higher plasma malondialdehyde levels, and Sargassum treatments significantly alleviated such increases. The HSD and HSD + SA50 fish groups significantly reduced plasma lysozyme, complement C3, IgA, and IgM levels and revealed higher intestinal aerobic and anaerobic bacterial counts than those at the LSD treatment; however, enriched tilapia diets with a high level of Sargassum diminished these negative effects. The HSD fish group significantly upregulated the expression of tumor necrosis factor-alpha (TNF-a), and downregulated interleukin 10 (IL10), catalase (CAT), and glutathione (GST) genes. Sargassum dietary administration, specifically at 50 g kg−1 level notably alleviated the adverse effects of high stocking rate on the cytokines. In conclusion, enriched tilapia diets with Sargassum treatment at levels ranging from 50 up to 100 g kg−1 alleviated overcrowded stress, oxidative stress, and immunosuppression caused by high stocking rate in Tilapia (O. niloticus).