With respect to the potential natural resource in the marine environment, marine macroalgae or seaweeds are recognized to have health impacts. Three endemic marine algae, Turbinaria triquetra (brown algae), Hypnea cornuta (red algae), and Ulva prolifera (green algae) that are found in abundance in the Red Sea were collected. Antioxidant, anti-microbial, anti-cancer, anti-inflammatory, anti-diabetic, and anti-acetylcholinesterase activities of methanolic extracts of these algae were in-vitro evaluated. Fatty acids and their esters from the extracts were determined using GC-MS. Extracts of T. triquetra exhibited the highest antioxidant activity with a DPPH radical scavenging IC₅₀ value of 1695.12±0.13 μg/ml and a ferric reducing power (EC₅₀) value of 574.71±0.15 mg/ml. The anti-diabetic and anti-acetylcholinesterase activities of H. cornuta extract were the highest, with IC₅₀ values of 19.89 ± 0.03 μg/ml and 16.97 ± 0.03 μg/ml, respectively. Interestingly, all extracts exhibited anti-colon cancer activity (CACO), with IC₅₀ values of 378.3±0.02, 276.88±0.08, and 274.32±0.05 μg/ml for U. prolifera, H. cornuta, and T. triquetra extracts, respectively. Moreover, the IC₅₀ values to human gingival normal fibroblast cell lines were 783.37±0.07, 692.7±0.01, and 124.36±0.03 μg/ml, respectively. These findings highlight the potential of these seaweed species for cultivation as a sustainable and safe source of therapeutic compounds for treating human and fish diseases.